If we go the full "death of the author" route, that only on-screen material is canon (and authorial intent is irrelevant), then Giger's intent and Scott's interpretation are both irrelevant and we can judge only what made it to the movie. The result? It's inconclusive: we simply do not have enough data, in ALIEN alone, to judge whether or not the creature inherits traits from the host. We do not see a non-bipepedal creature borne from a biped, nor do we see a bipedal creature borne from a non-biped, nor do we receive any narrative exposition one way or the other. So at the very most we can argue that it's inconclusive (which, incidentally, does not make Alien3 a retcon in the common sense; though it is "retroactively" introducing an idea into the "continuity" it is not contradicting the earlier films, which is generally what defines a retcon versus a traditional explanation.)
The second possible inclusion is if we allow authorial intent, in which case... Scott's intent is the only relevant one. I don't mean to diminish the man's influence, but Giger drew pictures. He did not create a narrative. He did not write the movie. If anything we should be deferring to the script and screenplay writers - O'Bannon, Shusett, Hill, & Giler (though Scott himself made substantial contributions) - to the best of my knowledge none of them have weighed in on this subject. As the man with final creative control over the project, if we are to take anyone's interpretation as the "true canon", it should be Scott's.
Scott saw what Giger created and decided to interpret it as an alien creature that inherits traits of the host, and he created the movie with that interpretation in mind. You cannot apply "death of the author" to one without also applying it to the other. And whichever way you'd like it, neither way is a retcon as neither approach contradicts the finished film.
My viewpoint follows the "death of the author" philosophy, always has -- I only follow what is on-screen (you'll find the exact same philosophy permeating my Nostromo thread
... and, incidentally, annoying pretty much the same people there
Thus, Giger's intent is just as irrelevant to me as is Sir Ridley's.
I only brought Giger up in the first place because someone used a partial quote out of context as an argument in favour of the circular logic at an earlier point on this thread, and the full quote I responded with mentioned Giger. The gist of the full quote being that his design preceded the idea of inherited traits. So, call it rationalisation or interpretation, but Sir Ridley's musings stemmed from the design already being bipedal
(which, strictly speaking, also makes it into a "retroactive construction", i.e. retcon).
For fans in this discussion then to point to the bipedal Alien as proof that the "inherited traits concept made it on-screen" IS
circular logic: (1 -- historical fact) the design is bipedal, ergo Sir Ridley speculates that traits might be inherited, and (2 -- according to these fans) the traits are meant to be inherited, ergo the design is bipedal. The circle is now complete...
My point is that the whole idea is caused by
what already is on-screen (or at least, on its way there); it does not cause
what is on-screen. In this regard, what is seen on-screen proves nothing
What, at the time, went through Sir Ridley's head -- or Giger's -- is inconsequential.
And yes, there is too little data in ALIEN to draw any absolute conclusions, I wholly agree. But the way I see it -- the creature is
bipedal, and balance of probability suggests that this is because the creature is
bipedal. As in, always
And in my game, this is the route I'm going -- as I've explained earlier: I draw the line after ALIENS. I do not incorporate any lore established in Alien3 or Resurrection in my interpretation. And as for those "prequel" ... abominations
... I will not even deign to mention them by name.
I do not in any way argue that others should do likewise. I do however maintain that others can
-- that cherry-picking elements from the lore as fits the individual GM's tastes and preferences is a valid choice
. I merely offer my ideas as a source for inspiration to the like-minded. Others are free to ignore them.