1 pointAs for missing tech from specialists, that is easy to determine. Specialists unlock some predetermined tech, like engineers and medics, and the Mk3 weapons of that Phase in a randomized fashion. That means some combination of specialists unlock some Mk3 stuff. If you have every Laser Mk3 weapon available, you did not miss out on anything regarding the Light Scout Op (since Caesans unlock Laser Mk3, while Sebs unlock Div Mk3).
1 point1. Those Xenomorphs you described are considered robotic and so cannot be stunned. Eggs? 2. There doesn't seem to be "successor" for Caesan light scout operator. However it does not seem he is very important for future. Quickly looking light scout operator seems to be somewhat bonus dude who can be replaced by some operators on phase 1. So probably no worries. So it seems you're right with that, no phase 2 operators seem to unlock light scout operator interrogation.