could an ainur be inserted in a newly created humanoid embryo in lieu of an original soul, or is that breaking several of Erus' laws, written and non?

The singular is 'Ainu', and should really be capitalised. However, where I was going with this was Huan, who is almost certainly a Power (I forget whether it's explicitely stated or no) and yet was born like any other dog (who presumably are not). So a Maia choosing to be born is attested. However, unless you care to count Luthien, who is debatably a Power herself... no, scratch that, she almost certainly is. Look at the last page of Of Beren and Luthien:

'These were the choices that he gave to Luthien. Because of her labours and her sorrow, she should be released from Mandos, and go to Valimar, there to dwell until the world's end among the Valar, forgetting all griefs that her life had known.

'[...] This doom [ie, mortality] she chose, forsaking the Blessed Realm, and putting aside all claim to kinship with those that dwell there.'

If this were a matter of standard rebirth, there would be no reason to live in Valimar: her uncle Olwe lives down at Alqualonde, and she'd certainly be more at home there than in the Golden City. So while it could be read differently, I suspect that this passage is confirmation that (assuming Ainur cannot beget Ainur, and you will note that despite all the couples of Valar, there are no children -- in fact, children were in the original concept, but were then removed) Luthien is an Ainu who entered the world to be incarnated as our dear Tinuviel.

As with everything else, of course, this is open to interpretation.

hS

I always had the idea that the Maiar had rather human (if much sturdier) biology. That Gandalf was described as an old man and needed to eat like anyone else seems to support that. When you say incarnations I assume you mean after they reincarnate? I kinda like the "released in a trickle via dreams". Have you read TADers? It does something very like this only with elves (and one Vala).


For that matter, could an ainur be inserted in a newly created humanoid embryo in lieu of an original soul, or is that breaking several of Erus' laws, written and non?

I don't think Eru ever said anything like that, though I could be wrong, but the idea doesn't seem like something he would endorse. Unless I'm totally misunderstanding you.