So a great number of those questions will get answered in Book 4 as all the groups and individuals you mention will be in the Abyss, at least for small parts of time and you will get to see what happens directly. Thus I shall refrain from saying too much here.
Dhampyrs are very expensive, which is why Lord Smerti’ wants his spawn back. There is some very complex wizardry required to create them, and it also requires an incubus (which is also expensive). Keep in mind, of course, that many vampires, or at least very powerful ones are wizards or animages and so can learn to do a lot of the needed wizardry…but then there is still getting one’s hand on an incubus, and in fact this is where they most often need to enlist others.
Knowing the wizardry to create a dhampyr, and having the knowledge, skill, experience to also do the complicated summoning of an incubus, once you know a name is very rare. Now of course, very old vampire wizards live long enough, eventually, to do this, but somewhat younger ones often team up or enlist other specialists.
If animus and antimus are like matter and antimatter, how can a dhampyr like Rede exist? If he’s half and half, shoudn’t he implode, or explode, or fall apart, or some other form of dying? I get it’s incubus magic, but that brings up another question. How can an incubus, made of whats basically pure animus, apply the antimus seed.
How will going to the abyss affect a Dhampyr? I’m pretty sure it was said somewhere that going to the abyss temporarily depolarizes animus and I think there was something about it maybe having a negative effect on undead, but I’m unsure since the undead army used an abyssal switch. But how would it effect a Dhampyr? I can imagine it doing nothing special, besides allowing them to ignore the need for blood, or killing them ( unlikely since Tizzy took Rede to the abyss), or allowing them to go fully one way or another, since their two halves are depolarized, when they repolarize there might be a way to influence it.
Are they not truly a form of antimus at all, but instead some type of neutral animus already? That could explain a lot, but I’m not sure if it has been contradicted in the book. If this is true, he might be able to feed off of antimus similar to how he feeds off of the animus in blood. Dhampyrs might be the key to defeating the undead. It makes sense to me, even if his animus isn’t neutral, he is half and half, and if he can feed off of one, he should be able to feed off of the other. The only issue will be finding a way thats not disgusting to him. Phaestus and Volund should be able to hook him up with something. It would be awesome if Rede could become powerful by fighting his father’s people instead of joining him.