While I see eye to eye with the Dimond Brothers probabably 90% of the time, I must disagree with their conclusion on this matter.  But then I disagree with the opposite side also.

Unfortunately, they make the same mistake regarding the Council of Trent that the Prots do with the Bible, not making necessary distinctions ... when the Prots say stuff like, "Well, the Baable says all have sinned except Jesus." (against Immaculate Conception) or "Call no man 'Father'" (against priests being called Father).

Yes, Trent teaches there is one Redeemer, but Catholic scholastic theology recognizes different causes and differeent agents of cause, some primary, some secondary.

Yes, Our Lord is in fact the Redeemer Alone, in terms of Formal, Final, Efficient causes of Redemption ... but this does not preclude God, by His Will. extending a cooperation in that act of Redemption to His creatures, by His will and His delegation, where they can in fact become Instrumental Causes.

There's also a failure to consider what is meant by "Co-".  Co- derives from the Latin word for "with", so it just means accompanyment, and does not imply equality or any kind of role similar to much less identical with His.

God gives grace, and we COoperate with His grace.  Does that mean that we somehow actively confer grace, or are the the cause of grace?  Of course not.  But we must COoperate for that grace to be applied to our souls.  So it's a passive role.

Now, Our Lady's role is more active, but it's still subordinate and instrumental.

Let's say there's a monarch, a sole absolute ruler.  But he decides to delegate his rule to some co-regents, other people that in certain capacities or territories might as well be the monarch.  We saw that in the Roman empire where you had four "Caesars" but one Emperor in charge of them (of course that started to fragment there).  So if the monarch delegates to these co-regents, does it mean that he's no longer the SOLE monarch, and that now these monarchs are somehow co-monarchs?  In a sense, yes, but in another sense no ... since these subordinates can serve only at the monarch's pleasure, and he can revoke that delegation anytime he wants.  So that's where the distinctions come in.  In a sense yes, and in another sense no.  It's also how you refute the Prot crap about "all have sinned", etc. -- and I'm afraid it's also where the Dimond Brothers fell short here.

Let's say I'm a rich guy in charge of a large estate with many people who live and work there.  I'm pretty busy, so I decided that I'm going to send someone out to go buy groceries for the household, plus I feel that these employees, servants, etc. should all contribute to their being fed.  So I decree that, well, if this person doesn't go out and buy groceries, you're going to go hungry.  Now this individual accepts.  So I, the rich guy, give money to this servant, and he goes to the store, does the shopping, and then exchanges the money for the merchandise, and brings it home.

Before going farther, that's precisely what God did, where He required human cooperation by free will in their Redemption, as it was human will that resulted in the Fall, and thus Our Lady's "fiat" was required.  God COULD have just created a human nature for His Son out of thin air, or "from those stones", as Our Lord said ... but He planned, in His perfect plan, to require human cooperation, and in that sense co-redemption, just like the servant above.  Had Our Lady said "no", which, absit, God would not have unilaterally gone ahead with the Redemption (except possibly if He found someone else, etc.).  Of course, she was so perfect that she would never have said no to His Will, but just hypothetically.

So, back to the metaphor.  This guy exchanged money for the groceries and came back with them.  Did he buy the groceries?  Well, yes he did, in one sense.  If you define buying as exchanging money for goods, then he actually made the transaction from the grocer, and as far as the grocer was concerned, that individual bought the groceries.  But he did so as an instrumental cause, since in reality, I, the rich guy, made it possible, provided the funds, gave him the mission / delegation, and then have lawful ownership of the groceries purchased.  So in one sense I the rich guy bought them.  In ANOTHER SENSE, my agent bought them.  Again ... distinctions, semantics, etc.

In God's amazing economy of salvation, He has willed that even we can participate in the redemption of others, in the sense of having the fruits of His Redemption applied to them.  We have no merits of our own, but the merits of Our Lord living in our souls through the Sacrmaent of Baptism, we can kindof help "direct" some of God's merits.  Perhaps one metaphor might be that we're like fire hoses that have a certain amount of autonomy.  We are not the source of the water, but God allows us to help distribute it and direct it.  If I pray for this person here, and make sacrifices for him, I'm a firehose that turns that water on the person.  I could be a crummy firehose, with a narrow diameter, and only send a few drops, or, as some saints, they could drench the person, etc.  In any case, the water comes from God's hydrant.

Yet, that speaks to a flaw that even many "conservatives" fall into, where, in doing apologetics against the Prot attacks, they fall into the trap of trying to say that Our Lady's role is identical in kind to ours, just in a greater degree, where her her role as Co-Redemptrix is just like ours, except that she exercises it to a remarkably greater degree, where it's just like ours, a difference of degree, not a difference of kind.  While they think they're appeasing the Prots, and maybe they do shut them up a bit, that's actually false.  Hers is a role different in kind also.  So, when the Prots attack us for "praying to Mary", the apologists try to say it's just like if I asked Nacy or Marge down the street to pray for me, just that Our Lady's prayers are more efficacious in proportion to her greater holiness.  False.

Our Lady has a unique role, different in kind from what we're able to exercise, since her cooperation of will was a necessary cause of Redemption, by God's Will ... whereas ours is not.  Yet at the same time, her role is also different in kind from that of Our Lord, an overlap or blend of the two, and thus intercessory.  In our participation in the Redemption, we could say no, but the Redemption would have gone on without us or without being applied to us.  Not so with Our Lady.  She was a necessary cause of the Redemption, albeit and instrumental cause, as she herself had been redeemed.

And this notion of there being different causes for the same action, or different degrees of participation in the outcome, so that the term "Co-" can be justified ... this is what is missed by the Dimond Brothers, where Our Lady's being a Co-Redemptrix (in one sense, as a certain type of cause) by no means contradicts the fact that Our Lord was the Sole Redeemer (in another sense, with different causes of the outcome).  There's no need for Our Lord being the Sole Redeemer to be a binary, false dichotomy.  It's just like the Monarch who select co-regents to help rule, by his own will ... without that changing the fact that he's the Sole Monarch.

So ...

1) I disagree with the Dimond Brothers, since Our Lady's role in the Redemption is different in Kind from that of Our Lord, and she was a differnt type of SUBORDINATE cause of the Redemption, and SUBORDINATE causes that themselves have been caused by the Primary Cause (as Our Lady herself had been Redeemed), they in no way derogate from the fact that Our Lord was the Sole Ultimate cause, with all secondary causes still deriving ultimately from Him.  They make the same mistake that Prots do here.  This chain of causality shows in St. Thomas' proof for the existence of God, where something that is a cause can itself be caused by something else, which is the Ultimate Cause.  Our Lady is a secondary caused cause of Our Redemption, she herself having been Redeemed by Our Lord, and her role in the Redemption is subordinate to that of Our Lord.

2) I also disagree with those conservative apologists who try to reduce Our Lady’s role to being different in degree but not different in kind from our own.

Finally, I disagree with this being a direct point of attack against the apostates in Rome.  If you read their docuмent, it just says that the title is not “helpful” because it requires too much misunderstanding, and can lead to confusion … the same type of confusion to which even the Dimond Brothers succuмbed, and that’s to say nothing of the average moronic Conciliar, 95% of whom could not pass a quiz on Baltimore Catechism No. 1.  Conciliar Catechesis has been so abysmal that I don’t think they’re wrong that 99% of the morons would not understand the term and then become even more easy pickings for the Prots, who will say, “See, we TOLD you all along that the Catholics worship Mary.  This proves it.”  Now, the Conciliars are in fact culpable for this, and so culpable for not being able to affirm Our Lady’s title for regular use, due to their negligence (likely on purpose) of catechesis, but what they said is in fact technically true.

I used to go after the Conciliars about annulments, but then a wise Trad friend pointed out that the Conciliar marriage preparation is so horrifically woeful that the vast majority of them do go in thinking marriage is temporary and does not come with an obligation to be open to children (even if you end up not being able to for some unavoidable reason … e.g. health issues).

I know there’s a lot of emotion vested since this involves Our Blessed Mother, but … the Conciliars have done far worse … and they are not without blame for this debacle also.  Unfortunately, there's also a fair amount of virtue signalling here, where people are vying to show themselves most devoted to Our Lady.  So, no ... if you were to declare Our Lady the 4th Person of the Holy Trinity, you would not be showing devotion, since devotion and love require truth.

I think the Dimonds issue with the term stems from their translation of Co+Redemtrix being "Female redeemer with". I don't know Latin so I can't comment on the accuracy here.