I assume the trans is still in the car , can't really see a lot from those pictures , there is evidence of a missed shift in there , don't know of any trans I have opened that didn't have that.

The wear on the fork is from driving around with ones hand on the shifter and putting a little pressure on it, that is not causing your shifting problem.

How is the downshift from second, when you can get it into it to first at speed greater than 5mph? Have you ever put it in 2nd, drove at speed and got on and off the throttle, if so does it pop out of second?

How did the oil look when it came out? Can you get some sort of magnet in thru the side cover into the bottom and see if there is anything metal down there?

For a test I would put a lighter oil in , maybe a mix of a GL4 oil and ATF and see if the problem still persists. If so your only choice is to drop the trans and go thru it, but it's probably going to be a little more than just changing the brass.

Pay particular attention to the condition of the syncro hubs, I rebuilt one that had ATF in it for I don't know how many miles, the cluster pin had significant wear and the inside of the slider hubs and the hubs themselves were all galled up and I spent a lot of time with a small stone cleaning them up, would have been easier to just replace them. You'll also want to take a dremel and small stone and dress down the teeth that the syncros engage , get points back on them.