Nice concept, reminded me of Messiah as well. I saw that softlock-issue is something you already considered, that's probably the biggest pitfalls for this kind of game.
Jumping felt a little finnicky at times, for example I consistently missed the jump from the ramp to the room with the multiple pistons. The character also doesn't follow the moving platforms correctly, when they're going down you "bounce" with gravity instead of staying on the surface, and occasionally the sideways-moving platforms tried to throw me off as well.
There are a few places where the camera clipped through the geometry, like at the roof of the room past the first locked door, or the vent section.
I assume you can only posess enemies that are unaware, but I haven't seen any indication of whether possession is possible or not, as the circle indicator still shows up when soldiers are shooting at you.
Combat feels fine, I imagine that's not the main focus of the game, though if you want to add dedicated shooting sections it could probably use some polish, as right everyone is rather spongy and there is not much feedback for getting hit. If you hold RMB a bit too long for picking up guns also makes the character drop them immediately afterwards as well. Currently reloading makes everyone T-pose with only their wrists moving.
I don't think the tutorial explains what's the top left bar is - I assume it's some sort of visibility meter for stealth?
Some better feedback for the puzzles would be nice as well. Some doors are only opened by some enemies, but since I didn't try opening the first locked door with the soldier it was not confirmed with me that the portrait shown after unsuccessful attempt is the character I need until I found the officer dude that you use to unlock the second door. I also didn't check the fans before getting to the console, so I only realized that was what turned them on after I went back to the game the second time.
Apart from the initial stutters at the beginning (which is likely shader compilation), the game froze for me once and crashed to desktop another time out of the blue.
Having sections where you're forced to stay in alien form due to size was interesting, perhaps giving the aliens some other basic abilities to mix things up could work well.
Best of luck for futher development!