Corpsecrafter et al. each apply to “undead you raise or create with any necromancy spell.” The Ritual of Crucimigration does not involve any spellcasting, only cursed nails, chanting, and the invocation of “evil powers and gods.” It is these things, rather than the ritual leader him- or herself, that result in the creation of the necropolitan. The ritual leader is just ensuring the proper timing of everything. Therefore, the feats that the ritual leader does or does not have do not apply, and again, since no spell was cast, Corpsecraft et al. wouldn’t anyway.

This was likely written this way specifically to avoid players easily getting substantial benefits at no cost to themselves.

On the other hand, desecrate does not have any such provisions. This implies that a necropolitan could gain the +1 or +2 HP/HD that desecrate offers to every “undead creature created within or summoned into such an area.” However, there is no indication that this is a permanent bonus: it is an effect of the spell. Once the spell expires, or the necropolitan leaves the area, the benefit is lost.

Necropolitan is a fairly high-power template. The benefits of the undead type are considerable. The costs are fairly considerable as well, but it is well worth considering for many characters, and dread necromancers benefit particularly well from it. That’s all fine, but piling extra bonuses on top, such as Corpsecrafter or desecrate, that cost you nothing, as they are aspects of whoever performs the ritual, that pushes the template beyond what I would consider acceptable. If I allowed a player to use these, I’d feel the need to offer ways to empower the other characters as well.

Which, of course, can be done and can be quite fun, but it’s definitely not an automatic part of necropolitan.

The Vampire would not heal

The way I read this (if I take your quoted text) is to break it down into individual sentences:

Hit: 7 (1d6 + 4) piercing damage plus 10 (3d6) necrotic damage.

This is clear; the bite, if it hits, does this much damage, and of course, the necrotic damage would be halved due to the resistance to necrotic damage that the Necromancer has.

The next sentence is:

The target's hit point maximum is reduced by an amount equal to the necrotic damage taken, and the vampire regains hit points equal to that amount.

Taken in isolation, since the Necromancer cannot have their maximum hit points decreased, the amount that it has been decreased is effectively 0, regardless of the damage taken as per the previous sentence. Therefore, the vampire would heal by 0, since the vampire regains hit points equal to that amount, which is 0.

Another way to interpret this is that since you cannot have your maximum hit point reduced, this whole sentence cannot apply, so rather than the vampire regaining 0 HP, the vampire simply doesn't get to regain anything since you can't have your maximum hit point reduced. Either way, they resolve the same way; the vampire gets nothing.

This also matches the flavour of the Necromancer's class feature, Inured to Death, which reads "You have spent so much time dealing with undead and the forces that animate them that you have become inured to some of their worst effects" (PHB, pg. 119); since they would have gained a certain resilience to their life force, they cannot have it drained out of them as a normal mortal could. Of course, they can still take damage...