First, it is common in informal spoken Japanese for syllables to be dropped, shortened, lengthened or changed in some way. In your example, you are correct in saying that 降りよっか is an informal, colloquial version of 降りようか. You might have noticed other 'slang' ways of saying things, such as 寒っ！instead of 寒い！ So it's not unusual to see syllables changed in informal Japanese.
About か、one function is certainly to specifically mark a direct question, but it has other functions too, such as indicating uncertainty or as a softener. In your example, 降りようか literally means "Shall we go down?" but in practice this is more of an indirect suggestion rather than a direct question. Here's another example of that same か：
(1) 今日は外食にしようか。"Let's eat out today". (Shall we eat out today?). You could just say:
(2) 今日は外食にしよう。"Let's eat out today". But it has a more decisive tone, like the speaker is clearly saying they want to eat out and wants the other person to agree. Sentence (1) has a more restrained tone because of the か、indicating that the decision would be more collaborative.