Cool ideas.
Anyhow, welding a chair to a turret base works, but it makes aiming very hard as it's very senitive.
Also, it would be great if infestor colonies would eventually grow into such sizes it could "deploy" some headcrabs, or something similar. Parasites what infest people, and if the infected can't find a cure, he/she dies, and from his/her body, a new colony will grow.
So a new NPC that gives people an "infection" then instead of dying how about they use my idea from before ( you become a spore and can fly around spraying your infestors everywhere). Your ideas are great though