Which certainly settles the debate that the creatures don't like fire. So isn't this all moot? The "lack of proof" from the first film argument you suggest makes no sense. No one had an opportunity to use the flamethrower. Dallas was sprung upon, Parker couldn't without killing Lambert, and Ripley did shoot it down the corridor prior to getting in the Narcissus. I seriously doubt R.S. was trying to play coy with the idea, it was just a plot device to give our characters and audience a sense of security, that they had a chance. Just because they never tested that in the first film was simply a creative choice. Ash wasn't being duplicitous, he would have intervened if the Nostromo crew had gotten close to killing the creature.
But the thing is, it didn't establish it for certain in the first film. I think the fire thing would have clearly been established in the first film for certain to make the point that it can be used agains the aliens (waiting until
Aliens to establish that detail for certain would be like Romero waiting until
Dawn of the Dead to establish that shooting a zombie in the head or causing severe cranial damage would bring them down). Yet, it wasn't. It was only suggested, yet not proven until Aliens that the bit with the fire was true.
Ripley had an opportunity to flame it when it was in the hallway between the Narcissus and her after she initiated the self-destruct. All she had to do was backup and and wait for it to come around the corner. And that bit you mentioned with Ripley using the flamethrower down the hallway prior to getting in wasn't her using the flamethrower. If you actually look at that scene again, you see it's coming from the other end of the hall (the way she came) coming at her, which is what drove her back and into the shuttle.
There's a lot of ambiguity in the first film that works in its favour. We never know how 'indestructible' the creature is because Scott doesn't tell us, and Ash is an unreliable source. It appears to survive the Narcissus engines intact, so who knows? Scott has also suggested that the alien's sluggish behaviour in the shuttle is because it is dying, like a mayfly.
Cameron had to resolve all that stuff to tell his story, but I think it seriously detracts from the creature's mystique as created by O'Bannon, Scott & Giger.
I never knew that about the Alien's behavior in the shuttle, or that it was dying. I always took it that the reason why the Alien was sluggish in general was because it was still a newborn and while the xenomorphs in
Aliens were faster because they were alive for a much longer timeline.
In a way, you're right about the character's mystic, but I think he did leave some wiggle room. Ffor example, though an M41A Pulse Rifle can cut through an xenomorph up close or at a distance, apparently regular 9mm rounds don't do much at a distance but work pretty good up close (Vasquez may have been able to kill one up close by pinning it to the wall and shooting it up close, but when Gorman tried to shoot one at a distance, the bullets appeared to be bouncing off of it).