I actually like the "going down 20 posts at a time", since there are many more topics with 20-100 posts than with 500+ posts. Any way to keep that function?
For us, go to last is a huge deal. Lots of mobile users and lots of very long posts. Go to last is a huge want from us.
The author also could create a new post stating that some changes have been made if it's worth an update in his opinion. I don't think any other concept is needed since editing posts is more wiki-like than forum-like. If your forum relies on something like this you might consider using a wiki platform instead or additionally.