Honestly in my opinion, mark the solution which helped the most unless its additional information.
It really is which one actually got you there in the end. This thread for example, mark the solution you most agree with and what managed to deal with your problem the most. As @starmaq stated you shouldn’t have to compensate for the helper using multiple posts in order to handle your solution - the best solutions are usually bulky and extensive to cover all instances.
If someone is confused by the post you marked as solution, they will probably scroll up and down to check what has been said in order to fully understand how to fix the issue on their end.
To be quite honest, a lot of programming issues are rarely revisited unless its a generic issue that many people have.
From your topics as key examples:
One someone is unlikely to revisit as its case specific:
One someone is likely to revisit as its frequently asked: