What year is the trans?
If it's 91-92.5, then I say go for it.
I just installed one of those 91-92.5 trans with rebuilt stuff inside, but I balked at disassembling it any further to confirm what sort of syncros it has.
In any case, the gear clusters need to removed and gears pressed off.
Upon reassembly, you have to check the preload on all the clusters and readjust if needed, likely yes.
I had to take mine to a trans shop locally, was best that way.
If the syncro is not working at all, the gear clusters are not mating at the same speeds when the fork is pushing the shell in place. This means your grind is the finely machined angle edges, or dog teeth that are being sheared off, worn down. Syncro worn means shafts at incorrect speed, dog teeth worn means improper cluster engagement and very hard to engage.
Get second replaced if things look okay once you tear down the trans. Given that you weren't able to shift nearly at all in your videos (2nd gear), I would be VERY surprised if the second gear cluster didn't need replacement to allow for properly timed shifts and long life.
Pulling apart what you have will yield results and you can send pics to a trans rebuilder for verification before shipping out individual shafts with clusters attached or ordering parts for a local to do the work for you.
There may be other issues inside the trans, and starting with what you have might be best, versus sourcing an unknown unit. I just went through this scenario for just a little over a year. My trans got a Quaife ATB LSD while it was apart, the AWD 25 spline model requiring the AWD DSM or KM201 ball and cage style diff bearings. This requires special shimming/Measurements BEFORE the bearings are pressed/installed to the diff. Please don't waste bearing like I did because they are slimmer in footprint compared to the tapered and separate race style. This requires extra shims under the bearing that goes between the bearing and the diff, below the final drive, when the two case halves are apart. More shimming may be required to get the diff up to the proper height to match the final drive gears, then when the case halves are installed to check for clearance and to calculate preload, there will be a new shim set installed at the top case. This is all fine and well for trans builders but not when you don't have bags of shims lying around or like my case, only shims to fit between the balls and cage bearing and the inner area of the diff housing, but no bag of shims to fit at the outer areas of the case on both halves! Fun times.
I'll step down from the pulpit now. Haha.
ive never seen all of these before lol 
