I get the no enemies on the right thing, but I have no idea how the Dimonds would not be schismatic when they're breaking communion even with other Traditional Catholics with whom they disagree.

Because they believe them to be manifest heretics either because they deny EENS (or one of the other Catholic dogmas concerning baptism) or they support and remain in communion with manifest heretics such as Jorge Bergoglio, etc.  It is a Catholic dogma that Catholics cannot participate in non-Catholic services (e.g. Novus Ordo sect services).  It is also forbidden for Catholics to support heretics.  I think one thing they may have gotten wrong is that Catholics are required to break communion with the heretic but not with Catholics who didn't break communion with the heretic.  See https://romeward.com/articles/239752007/heresy-in-history  (in particular the episode concerning the different reactions of Hypatius and Eulalius to the heresy of Nestorius).  But otherwise MHFM has the principles exactly right.  You could certainly argue with them on points of fact.  Such as whether or not a certain Catholic who indirectly denies a dogma is formally guilty of heresy or is simply a Catholic in error.  In the later case, there is no need to break communion with them although there certainly is an obligation to correct them.  I see MHFM as suffering the fate of all bearers of bad news.  Rather than deal with the problem, people tend to shoot the messenger.

Precisely this.

I don't consider myself to have big enough shoes to write off anyone who professes fidelity to the Roman Pontiff --- even if the last Pope they recognize is different from the last Pope I recognize, we just differ on concrete facts, not the concept --- and I condemn no one, nor do I "read them out of the Church", not even relatively conservative Novus Ordinarians.  Not the SSPX, not sedevacantists, not the Dimonds, nobody.  Things do start getting hairy with way-out-there groups such as the Palmarians and those who just explicitly say "we are not affiliated with the Roman Catholic Church" (I seem to recall one independent bishop, who celebrated the TLM, saying that), I'd probably draw the line before we get to them.  But not otherwise.  We can all fight like cats and dogs over the matter, but at the end of the day, no one really knows, and we will not know until we have a fully traditional Pope who declares on the matter.  In the meantime, to paraphrase the Episcopal non-priest William Porcher Dubose, it is like walking barefoot at midnight across a barnyard full of chicken ordure (only he did not say "ordure").

What if they think the last Pope is Peter.

Look, to be clear, I'm not writing them out of the Church per se.  I have no idea where their souls are at.  But objectively speaking, if you're breaking communion even with Traditional Catholic clergy, I think that's objectively schismatic behavior.

What if they think the last Pope is Peter.

Look, to be clear, I'm not writing them out of the Church per se.  I have no idea where their souls are at.  But objectively speaking, if you're breaking communion even with Traditional Catholic clergy, I think that's objectively schismatic behavior.

That is a reductio ad absurdum, but I see your point.  If I were pressed, I would have to say that if someone professes the same belief in papal primacy as traditional, orthodox Catholics do, and that they believe what the Church teaches on papal primacy, then I would need to hear their reasons for maintaining that [fill in the blank] was the last valid Pope.

There is that guy who styles himself a cardinal, name escapes me right now, "Don" something, who says (if I'm understanding correctly) that Pius IX is the last legitimate Pope because he was the last Pope to reign over the Papal States.  And then there is Richard Ibranyi (RJMI) in New Mexico, who (again, if I'm understanding correctly) maintains that there have been no valid Popes since around the 12th century, that Aquinas was an apostate, and that scholasticism, and philosophy itself, is inimical to divine Catholic faith.

It gets really hairy out there on the fringes.

MhFM’s Brother Michael Dimond Plagiarizing St Alphonsus Liguori • BitTube.tv

This guy is cringe. He literally felt the need to respond to a video from them about how the Bible proves Papal Primacy. He is happy his former MHFM supporter son defected to Eastern Orthodoxy. Who cares if the Dimonds quoted St. Alphonsus Liguori in a video describing the pains of hell to help people amend their lives?

Does Cryptinox think JP II is the antichrist too? LOL
Shine On You Crazy Dimonds, Part I: MHFM's Literally R*tarded Eisegesis of Scripture - YouTube

I'm in-between on the Dimonds.  I do believe that their behavior and attitudes are OBJECTIVELY schismatic, but at the same time I do not hold them in contempt like so many do, practically spitting at the mere mention of their names.  Charity should extend also to the uncharitable.  I do not agree with them about everything, but they get a lot of things right, and I'll give them credit where credit is due.

Your approach much mirrors mine.  I have two or three of those magazines they published 15-20 years ago, one IIRC showed Our Lady of LaSalette weeping.  As in all things Catholic these days, I "take the meat and leave the bones".  In these confused times, people will inevitably disagree on what constitutes meat, and what constitutes bones.

And if in eating the meat you choke on a bone, then what? If not you, then what of the others less knowledgeable (who knows how many?) who choke on some of the errors they preach? That danger is present there, and it is sometimes mingled within whatever truth they teach. It should be noted that by their air of authority, they teach just as if they're commissioned to teach by the Church, and many unknowing accept them as such...

Like the Index of Forbidden Books, I admit they do have some extremely some good material, but as Our Lord warned, that only makes the DBs an even greater danger, maybe not for some, but it is for many.        

A bit of comic relief (or at least attempted comic relief!) might be in order here --- it's like eating rabbit or catfish.  When you eat rabbit, it tastes like chicken, but the bones aren't where you'd expect them to be --- it's weird.  (Ditto for goat.)  When you eat catfish, the bones are small, pervasive, and you have to be extra-careful eating it, lest you swallow a bone.
I don't recommend the Dimonds to the average, only moderately catechized Catholic. I can read them and sort out truth from error (or at least gross imprudence).  Some cannot.  It is not literature I would leave in hospital ERs and bus stations, let's put it that way.