1. For the Daemonettes, not entirely sure what your options are at this point in OpenXcom, but I think if you want to keep the Daemonette seduction, I would either make it a melee only low TU cost temporary Mind Control that is pretty much guaranteed to work, or increase the TU cost of the Chryssalid style melee substantially, while giving her a low TU cost alternate melee that just does normal damage (much lower than Daemon swords) and morale damage. Being able to instantly corrupt at low TU cost feels more like something a Keeper of Secrets or Great Unclean One would have (or Lord of Change in terms of mutating their victim into a Chaos Spawn). Ultimately, players probably shouldn't be way more afraid of these enemies closing to melee than a Bloodletter.
2. I do see that the Hellswords/Plagueswords are 150 power. Honestly would stat the Bloodletter sword as per a Power Sword, and drop the Plaguesword to 90-100 power, but it deals damage to Energy and Morale in addition to inflicting many extra wounds like other plague weapons, which means it'll be real bad if you don't have an Apothecary nearby to treat the victim immediately.
3. I understand the puke being a contingency; I do think it would fit the theme of the Nurgle followers to have it inflict many extra wounds as I assume this symbolizes the illness it inflicts, and perhaps Energy/Morale damage to represent the impact of infection. Rest sounds good.
Other things that come to mind is that Terminators (including enemy Terminators) should probably have shields of some kind (their refractor fields), and the Chaplain Crozius is pretty insane with max Bravery, namely because two shots of it have the capacity to cover most of the map in fire, making these guys deadlier than my elite Librarians. Might consider capping the AoE size in exchange for it being more powerful, or having other effects.
But yeah, so far so good, been really enjoying the mod! Just about to do the final mission.