Q1) is it appropriate at unix.sx to ask OPs to post answers once they found them by themselves, and to mark them as accepted?
Sure we do this all the time. Many of us often will also fill in gaps that we stumble across on the internet or on this site and will post a question, merely to provide our own answer(s) a home here and on the internet at large.
It's always appropriate.
If a user expects to get free answers and give none when they figure it out, well I'll let you finish the sentence. More specifically:
2) I've fixed quite a few new things by seeing how older related things were fixed. I think there's value in deleting barely anything.
An answer is an answer. It doesn't matter the source.
The purpose of stack exchange is the collation of good questions and answers to them.
The individual poster getting their question answered is a beneficial side effect.
If you have a noticeable problem to solve, typically you will continue with it in parallel anyway. Things you discover you should ...
It's always appropriate to answer your own question if you have the answer. If the asker signals that they have solved their problem, it's appropriate to encourage them to post an answer.
The answer must explain how the problem was solved. Don't answer “The problem is fixed” just to make a question answered. Explain what was needed to fix it, e.g. modifying ...
General Linux questions are allowed on both sites. We usually point people to AskUbuntu if the question is Ubuntu-specific, and they point people to us if the question doesn't apply to Ubuntu at all. In your case, all of those questions would be fine here, although you might want to put Repository to upgrade Sigil to latest version on (K)ubuntu on AU since ...