I'd suggest here, assume nothing here, treat it all as 'suspect' until checked out.
First, see if the problem is in the Shifter itself or in the Gearbox. I'm suspecting there could be an issue in the Shifter given that he 1st and 3rd issues are on moving the Gearstick forward. It maybe worn in a way that when 'hanging' under the car, there's something that fouls up, so be mindful of that when checking it out.
I doubt there's an issue with the Clutch given it goes straight in to Reverse without Grinding.
I doubt worn Selector Forks are the issue here. Usually, when they are worn, it won't fully select a gear to the point where the Synchro Hub Detents are holding and so it will jump out of gear. After doing this a few times, you still need to replace the selector fork and a gear too, so, as expensive as they are, if it's out of spec, it's needed.
Just thinking out aloud here, if the issue is internal to the Gearbox, the Rod that the Selector Forks are mounted on could be bent, but given it was OK at first, this is not likely, however, refer rule No. 1 - assume nothing.
The Paddle on the Shift Input Rod could be loose, the Syncho Hub itself is worn. When this happens the outer track tends to lean over instead of sliding. There could be something foreign in the Gearbox.
All the same, I'm gravitating back to the Shifter here. I can't think of anything (other than a Bent Selector Fork Shaft) that's internally common to both 1st and 3rd gears.