I'm not a lawyer, only a bit familiar with CC licenses (I read them and read a bit of stuff about them).
I agree fully with the answer of Ryan M, it's perfectly valid to take part of other posts and cite them with attribution even in community wiki posts. Just want to add some details here.
The provision for attribution of CC-BY-SA is detailed in section 3(a) of the license. It tells you what information you have to provide in 3(a)1, that you have quite a lot of freedom how you provide it in 3(a)2 and that the original author has the right to request removal of that information from you in 3(a)3.
Obviously the software running this platform won't help you like it does when doing edits (but otherwise it's completely the same mechanism), so whenever you copy parts of other posts in your post (community wiki or not) you must give attribution in any reasonable way. Typically you will do what Ryan M proposed. You don't have to do more.
The fact that it's a community wiki post is not really relevant here. It's just relevant for you. What are “Community Wiki” posts? says that they partly transfer ownership. Not sure if they meant that in a legal sense. I could imagine that users creating community wiki posts still have users as authors, but these users just dissociated according to section 3(a)3 (the right not to be named). But then in the edit history of community wiki posts they are still named and usually you would request that separately so maybe not even that.
Finally, it's probably not relevant here because if you split a post you probably created an adaptation but if you would just copy a whole post without any further ado it could also be just a reproduction instead of an adaptation. In that case it might be wrong to increase the CC-BY-SA version (as the timeline would indicate) because section 3(b), the sharealike part only applies to adaptations. I'm not familiar enough with copyright law to say exactly where the difference between mere reproduction and adaptation really is defined.
See also Is usage of a still frame from a video considered a derivative work/adaptation?. The question is if splitting the answer from a question is an editorial choice and therefore an original work or just a mere abridged reproduction of the original content.