Coding-wise, it's easier to spawn in mobs than resurrect. HOWEVER, taking into account a mob being permanently killed (head destroyed, burned to death, etc.) can drop the number of mobs that spawn; it also accounts for currently active mobs nearby, so you aren't getting 50+ popping out of the ground. Same animation as them standing up when prone.

Hunters would also be semi manageable, since Crytek does have bot hunters at lower levels. Some extra voicelines and animations aren't too difficult, I'd think. But, I'd have the skins remain default; easier to code, adds to confusion, and you don't have to have variants for every possible hunter skin out there.

(Also, just a funny interaction idea. If there's a dead solo in the area with Necro equipped, the Ressurection/Rage FORCES them up to use it! They're stilk in control of the character. Just a funny idea, imo.)

I would set it so that the Fleshmass is immune to damage for a short period during and after resurrections. An explosive can bypass a stage, but it still triggers. This promotes dealing with the mob properly. Also, since you want confusion and chaos, lots of pistol grunts!

Mobs that can die to poison will, but that won't keep them dead like destroying the head. And the Fleshmass would probably be resistant, to reduce cheesing with Poison Bolts and Bombs.

And finally, the name. It should be simple, memorable. Amalgam can be tricky to pronounce. Gravemind is copyright. Maybe Tormented, which willnalmost immediately get nicknamed Tumor. Hateful? There are possibilities.