Newest version of Pumpa has ability to edit posts. Should be in Debian sid very soon

Yes, but for an HTML post, it's a plain HTML code editor.

Yes, that is unfortunately harder to fix. Pumpa uses Markdown for formatting, and posts are stored as HTML in pump.io. I would need to convert back from HTML to Markdown in order to do that, and that's more or less impossible or at least extremely error prone.