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.
So, the "-trix" at the end is little more than a gender suffix, just as we might say actor and actress ... really the same thing per se, or in essence, just one happens to be a male and the other happens to be a female, but the suffix does not change the root of the word.
Certainly the "co-" can cause misunderstandings because in many English (and other modern languages) it can in fact connote a joint causality. I am the co-author of a book.
But in Latin, the "co-" or "con" can but usually does not have that sense.
So let's look at the word Redeemer. -er suffix refers to an actor or an agent, known in Aristotelian and scholastic terms as an efficient cause, a doer. So a Redeemer is the cause of Redemption.
But recall that in the same philosophical framework, causes can actually cause other things to cause. In other words, there's often a causal chain, with a Primary Cause and then a Secondary Cause, etc.
If I throw a baseball through a window, I was the sole cause, and am the responsible party. You can't blame the ball, since I'm one that caused it to become a cause of the broken window. I set the whole thing in motion, and the ball was itself a caused cause. That's related to St. Thomas' one proof for the existence of God as the Uncaused Cause. This secondary cause is also known as an instrumental cause, but St. Thomas et al. referred to secondary instrumental causes as "concausa" ... see that word "co-" and "con" again.
So, while in Latin the instrument cause is a CON-cause, in every real sense of the word, I was the sole cause of the window being broken, since I caused this secondary cause to do it.
See how in Latin, the co- or con can easily refer to a caused cause or secondary cause even if in English you often have this notion of equality.
Let's say I am the author of a book. I pick up a pen and write the book. In English, it would be stupid to say that the pen was a co-author of the book. Nobody would say that, but would agree that I wrote the book alone and had no co-author. That refers to the individual who was the primary cause of all its contents. But in Latin, you could actually say that the pen was a "concausa" of the book having been written. In fact, it was a necessary cause, a sine qua non, since without it I could not have written the book ... and so it made a very significant contribution to the effort.
Getting back to Redeemer, as the CAUSE of our Redemption (the agent or efficient cause). If Redeemer is a cause, a primary efficient cause, then a Co-Redemptrix would be a "concausa".
Redeemer = "causa efficiens Redemptionis"
then the con-"causa efficiens Redemptinis" would translate to a Co-Redemptrix, even though she's a secondary efficient cause, an instrumental cause, and a caused cause of our Redemption, herself having already been Redeemed by Christ, which enabled her to become that instrumental cause. It's Christ's Redemption of her that caused her to become an instrumental cause of our Redemption, just like I caused the baseball to smash the window, and being the Primary Cause and the Cause of all the Other Causes, I am the Sole Cause of ALL the effects that I caused, even if I happened to use an instrument to effect some of those Causes, by my choice.
So God can made other causes required, not because they're absolutely required, but ex suppositione finis, determining that it should go that way by His Will. God could have created a human nature for Christ from thin air or "from these stones", but He required that there be the cooperation (not co- again in a subordinate sense, our main English word with that connotation) from the free will of a human person, since the free will of a human person was involved in the Fall (note that Our Lord is a Divine Person). So God ordained that Our Lady be a necessary cause, a sine qua non, of our Redemption, and also for a greater perfection of His plan, ad perfectius agens.
So, St. Thomas and others all say that Adam was the principle cause of our Fall, but we all know that Eve contributed mightily to the situation and was a cause also, yet a secondary or instrumental cause, in this case instrumental for the devil, who then used her to cause the Fall of Adam. So the devil plotted (final cause) to use Eve (instrumental cause) to have Adam bring down the human race (primary efficient cause). God, then to reverse this planned (final cause) to use Our Lady (instrumental cause) to have Our Lord redeem the human race (primary efficient cause).
Recall that the word "redemption" comes from the Latin "to buy back" or "ransom", connoting a "purchase". So take the following situation. I am a rich man, lord of a manor. I tell one of the household that if he wants to go out to the store to buy gifts for everyone, I would give him the money and send him out. But, because I insist on their cooperation, I'm not going to go out on my own. If they're too lazy to help, then they don't deserve anything. This member of the household agrees, so I give himthe money and send him on the mission. He goes to the store and buys the stuff, and then he comes back.
So, who "bought" the merchandise? Well, in one sense I bought it, since I came up with the idea, formulated the plan, and provided the money. Yet, in another sense, is it wrong to say that this guy went to the store and bought the merchandise? No, since the definition of "bought" refers to exchanging the money for merchandise. Of coures, it was the master's money, but nevertheless, he put the plan into action, acting as his agent, as an instrumental efficient cause.
In Latin, then he was a "con-causa" of the purchase, and therefore a co-purchaser, even though I alone formally purchased it, and I'm the one that caused him to go purchase it. So I caused all of the effects in the end, even if in some cases I caused another cause to cause some of the effects.
But ... I do agree that in modern languages this term could cause confusion, especially those not particularly well versed in Aristotelian / scholastic philosophy, where they will cleary consider the term to refer to Our Lord and Our Lady being joint primary efficient causes, which is not correct.
So that's why I barely disagree with the Modernists on this, and the Dimonds cited pre-V2 theologians who said the same thing, that in practice it might be best to avoid the term lest it lead to misunderstandings.
Back to Our Lady. Just as God made Eve for Adam as a COM-panion, so too in another sense Our Lady was Our Lord's closest COM-panion during His Redemption of mankind, so that's another way in which one might undrstand the term Co-Redemptrix, as she was His Companion in the Redemption, closely accompanying Him (sorry for the Novus Ordite term there).
So the term absolutely can be legitimate, if properly defined and understood ... but it also cause problems, ESPECIALLY due to the abysmal catechesis of Novus Ordites, who would become even easier pickings for the Prots. "See, we've been telling you all along that Catholics put Mary on the same level as Jesus." Which Novus Ordite, 99% of whom couldn't pass a quiz based on Baltimore Catechism No. 1 is going to explain secondary efficient (or instrumental) causality. "Sorry, Mr. Protestant, but that term simply means that she was the caused cause, a secondary efficient, aka instrumental cause, of our Redemption, and the primary cause causes the secondary cause to cause the effect, so is ultimately the cause of all the effects even if using various instrumental causes to bring them about. In Latin, co- does not necessarily refer to equality or some share in the primary casuality." 90%+ of Novus Ordites still think the Immaculate Conception refers to the conception of Jesus.