But ... this is what I think ... the moment your xenomorph is too easy to kill or too easy to neutralize ... that moment your game becomes less horror and less stressful (I would even say pointless) ... why ... because the xenomorph is the heart and soul of this horror / sci-fi / gore game.
You lose all the momentum when your xenomorph becomes weak. The xenomorph is your lead ACTOR.
That's my point of view. That's the strenght of my game.
I agree with your premise completely, hence why I emphasized a Game Mother should never let the players get that comfortable - either there needs to be more critters than the players can comfortably manage or they need to act in ways that always keeps the players on the back foot. To draw an analogy to the recent video games, I believe combat encounters with xenomorphs should feel like
Dark Descent rather than
Fireteam Elite - that is to say, careful and tense with everyone fighting for their lives vs. human lawnmowers enacting a power fantasy.
As for Grappling with a xenomorph, with RAW stating that only humans and androids can be Grappled, I wouldn't even entertain that idea unless the PC
was in a power loader like Ripley. The film fight is tricky to put into in-game mechanical terms for a variety of reasons (mainly that nobody in 1986 was worried about it translating into a TTRPG system over 35 years later), but I reckon an opposed Strength check (maybe allowing Close Combat to be included on the PC's side, otherwise the power loader is basically set dressing)
would be a good way to go about it. If we're talking about the scene where the Queen is literally pinned down and unable to move underneath a large piece of industrial equipment at the bottom of an airlock shaft, I personally would rule that the Queen would have to spend at least one of her initiative turns using two fast actions to get free - one for a Strength check to push the loader off of herself (which I would probably set at a -2) and one to stand up. The situation is still tense and threatening and there is still a strong possibility that Ripley in this scenario doesn't survive.