Sounds like the PO was a mechanic wanna-be.
The rockshaft looks so easy to remove doesn't it?? It's not. While I have never needed to remove one, several people on here have..... sometimes they end up cutting it to get it out. Your a machinist, so maybe you will have more patience, and know how to properly "heat and beat". You know the story... most guys heat too slow, then heat too much, then beat too much, then heat again. By that time everything is so hot and swelled up it wouldn't matter if it was a slip fit, it ain't movin'. Drive the pins out, sand all the paint off, soak it in Kroil and come back in a few days. Bet it comes apart. If not, well.... you know what to do.
On the seal, my gut says it was installed crooked. Lasted as long as it was going to. If it didn't leak from the get go. I never want the new seal riding where an old one was so I try to get it in a different spot. Works good when they give you a 3/8" thick seal with a 3/4" deep groove.
I use anaerobic sealer on the outside of seals most of the time. Usually helps hold them in place.
The carb kits for those carbs are kind of weird when you get them. Is your an adjustable jet, or fixed? I think the carb's with the fixed jet don't have a removable needle valve seat....
Overall, sounds like you are going through it right.
