Well, a couple things on this:
-40 HP shouldn’t be that hard for the party to heal up in 4 rounds (3 rounds of no combat that Battle Trance is still up, plus the 1 round to heal before death). It’s pretty rare that a player will have no attribute accessible to healing at all, and between them all in those 4 rounds, should be able to do it.
Deathless is a bit unique, and being -40 is different from being at 0. I’m not entirely sure as a GM I’d allow Divine Agent to work with Deathless or not. If it is a constant thing recurring, probably not, at the very least I’d require a roll to see if Odin would, or if Odin is satisfied in the warrior death and accepts them in Val-hall, etc.
Whenever players purposefully circumvent something, I’m pretty leery about it. So the players doing a finishing blow just to try and get them out of the deathless situation, not sure I’m a big fan of that. I’d probably also make that not work or require some sort of roll
Deathless says you have to be healed, technically Divine Agent isn’t healing, so you could probably make an argument that it wouldn’t work unless ALSO healing happens after that.
Divine Agent works AFTER a finishing blow, so you are just at 1 HP and don’t take the damage from the finishing blow after being raised up to 1 HP, so you wouldn’t drop to 0 HP.
RAW, I suppose it would work, though like I mentioned, a possible case that it doesn’t since it isn’t healing. Up to the GM in the end for the situation and if they feel it should work and be rewarded.
It might not be too powerful to allow in the end, since they probably do this sort of thing several times in a session, so it would only work once. It is one of those things you’d have to actually deal with it in play to find out if it is too powerful.
I imagine narratively, either way, there would be some repercussions from doing a finishing blow, especially multiple times.