"There are stairs and an elevator. Which do you prefer?"
I see 「〜どちらのほうがいい？」 more like "which do you find to be objectively better?" rather than 「〜どちらがいい？」 as "which do you find to be preferable in this given instance?" But I am not a native speaker so please take this with a grain of salt.
Why does どっち get a が after? All the other question words do not, if i understand correctly
Because this specific question word is acting as the subject of the sentence: it is the one which "is good/preferred".
Here the structure is the same as in "something is good", but changing "something" by "which" and turning it into a question
I always see GA as the one in which one.
Though totally wrong, till now it worked for me.
If they insist on using 丁寧, might as well use it on all the words, not just the verb. Also, doesn't が fit better then は?
階段とエレベータはどちらのほうが好きですか？ Also works right?
The most natural way to say this would be 階段とエレベーターと、どちらの方がいい/好きですか。
どちら here is the subject, not the decision between the staircase or elevator, so the options do not get a は.