Your first sentence is missing a
the in front of
Assuming that is in, the the one that is more correct is A because both tenses match (
Remain on the chair until the lift has stopped.
Although, I would probably rewrite this sentence as:
Please remain seated until the lift has stopped.
For the 2nd sentence, B) is correct, but A can be rewritten to say:
Do not attempt to leave the lift until the landing gate is fully open.
Like the above, both of the tenses need to match for it to be correct (although in the 1st set of sentences,
stop wouldn't make sense because
stop is not an adjective unlike
I do prefer the "fully" to precede the word "open" in both cases.