I've only seen a broken shift fork (the small ones at the lower ends of the shafts) once in a Mitsubishi transmission - but if this would be the case one of the gears would be stuck and the gears using the other 2 forks can be engaged, resulting in blocking the transmission.
The shift lever on the gearbox is pressed onto the shaft, but have a look if it slips over the shaft when moving in the direction where gears 2,4,R would be.
But if this would be the case gears 1,3,5 could be engaged but not disengaged.
Can you hear the shift ring locking onto the gear when moving intp 2/4/R?
I've seen a broken 4th gear wheel once where the shift gearing was separated from the gear wheel, but even our oldest mechanic couldn't remember of such a failure in his 40+ years. So having 2 gear wheels with this failure would be near to impossible...
How hard did you push to force in the 2nd gear? Mitsu transmissions are one of the smoothest and easiest to operate, the only reason you need to force in a gear would be way too high rpm/speed for the gear, wrong or no oil or completely worn synchro rings...