Cannot select any gear even with engine off or idling (6MT)

[sapote]

All of them please

3 sets of spring/ball (later cars have spring/sleeve instead) at the top rear of the box: they are the detents to hold the internal shift forks for R, 1/2, 3/4 gears.
1 set of spring/ball at the left rear box to hold the internal shift fork for 5/6th gear.

I don't think they are the cause of your issue.

1 roller pin under the GPS at the left rear of box. This is the detent for holding the internal/external main shift shaft. This is the one that I think causes the issue: the roller pin is damaged.