Is it possible to view previous versions or edits/revisions of questions, like on Wikipedia?
Yes. If a post (that is, question or answer) has more than one revision, you can see the list of them by clicking on the timestamp of the most recent one, which will be listed beneath the post. If it has only one revision, the only timestamp displayed will be that of the original post, which will not be clickable. To demonstrate, I've edited your question (this one). However, note that multiple edits by the same user within about five minutes and without intervening edits by another user are collapsed into one edit (at least as far as users can see).
If a comment has more than revision, you can't see the old ones. But a little symbol will appear at the end of the comment indicating it's been edited. I'll comment on this answer so you can see.
Absolutely. Try looking here for example. You can click on each entry to expand them and view previous versions, as well as comparing the current one side-by-side. The way to get there is click on the word/s after "edited:"