Those sound like fairly typical symptoms for transmission issues. The bad shifting would be bent shift forks and or worn dogs. The humming is a little different, but could be gear dog related, I'm not sure.
The good news is splitting the cases in your garage is doable (lots of things to be careful doing, but doable), and complete transmissions on ebay are affordable and common.
Try everything else first before splitting the cases... but objectively, that sounds like your problem.
On the clutch side, you can pull off the inner and outer cover without splitting the cases, and you can remove that shifter shaft and inspect that part of the assembly. Though I would think that if that was screwed up, every gear would suffer, not just 4th and up...