It was established that after a certain period of possession, i.e. prolonged posession, the Protodevelin could no longer seperate themselves from their host body. Basically, it was too late for Gunther & Gigile's host, otherwise various Anima Spiritia exposures would have knocked them out of those bodies (as Gabil was knocked out of Gamlin). Same goes for the Evil bodies, it was long since no longer possible to "give them back."
Sivil only possesed Akiko for a limited time, so she was safe. The real question however, was Gabil someone who had been posessed? Or were Gabil/Glabil originally designed to have a "remote unit."