"... biting and lunging at them when they walk by..."
I just want to clarify that your kitty is not merely playing but truly defending territory? So he does not do this to you and is normally an affectionate cat?
If these are territorial attacks one suggestion is to start all over with the introduction process. Confine kitty to one room where he has all sorts of cool toys, food and litter. Confine him to this play room when you have houseguests and also occasionally when you are alone with him so he doesn't associate his play room with punishment.
When fiancee comes over assign her to be in charge of feeding him. Initially she should say hi Kitty before opening the door and just bring his food in, walking with a confindent style so as not to provoke an attack. Then leave the room -- no petting or attempt at interaction. Gradually as his tolerance for fiancee improves she should try to sit quietly in the room, for longer and longer periods. Not soliciting attention from him -- just reading quietly. When he is ready he will begin to associate her with food and become curious and want to spend more time with her. Then have each of the kids be assigned feeder role and do the same thing over with each kid.
Blending families can be a really stressful time. . . it can be done but many times it is a slow process. Good luck
PS I have allergies to cats also. Washing my hands after petting and vacuuming really have helped.