Well... Now I am tempted to tell Eko that we are a necromancer. It should be possible to convince them that we want to help, and that we are not his responsibility, and that we want to our magic to help study the undead without revealing which element our magic is based upon. And Nightroar presumably already knows how to bypass the wards, so us finding out wouldn't create any problems that don't already exist. But, taking a broader perspective, it really is very much too much of a long-shot to be worth the risk. Eko is obviously conflicted, which means that they still have some loyalty to established protocols...
As for the forest, I am all for trying out some experiments, we can enter the forest safely enough, so long as we don't go too far into it. The forest seems to be containing the entities, so controlling one and taking it out of the forest might end up releasing it, or destroying it by severing it from its source... But if we can control one, and examine its energy flows in detail, we may be able to figure out how it manipulates souls. It would also be very useful to know if a single steal vitality can destroy one, because we may end up being stuck between the forest and something worse...
As for the soldiers, we could watch them, they are likely to notice elves if we don't hide it, and the villagers will likely mention elves so... We could claim to be investigating the undead threat on behalf of the Life nation... Present ourselves as a scholar and an escort. If they discover that we possess magic, then claim that we were hiding the fact so as to avoid drawing attention to ourselves, as strange people are a curiosity, but strange magic is a resource and will attract more dedicated attention. If they can sense necromancy on us, then suggest that it might have come from the children that we rescued. If they seem to know that we are a necromancer, which would pretty much be a game-ending power at this point, then we deny it until we find someone with more curiosity than righteousness...
P.S.
I support remaining in town for another day. Running would look suspicious.