1) is there someplace I can get a replacement stop or do I need a whole replacement trigger guard in order to get that part?
www.globaltrades.com/ak_receivers.htmlScroll down the page -- they have the stop for $6.99.
2) is there a better way to do this? I checked out Firebase's thread but I don't have a drill press so I'm winging it a little.
Carefully. I grind down the rivets with a dremel bit. You need a dremel type tool for these builds. Harbor Freight usually has them on sale. After grinding flat you can drill and then punch them out.
3) I haven't gotten the looooong rivets out of the rear trunion yet--how should that be tackled with no drill press?
Slowly and carefully and from both sides. You could also get a small drill press. I'm sure you'll find hundreds of uses for it over the years.