Alright. I just tried dying and re-stealth in two separate reloads after having injected the mutagen. Still the game crash on both occasion. Luckily, it didn't crash after killing the final boss, most probably thanks to the combat end by the 'cinematic scene' of the final boss dying. Even then, there's a left over doppleganger from one of the tentacles's Bilocation, which initiated combat but then disappear, and I can end combat without crash.