As far as I know anything about this area, the adjustment of the linkage consists of loosening the mounting bolts on the vertical sides of the central tunnel either side of the gear lever, and juduciously sliding the lever unit back or forwards slightly until you can select all the gears. In my own experience the movement needed is quite small, perhaps only 5mm or so to make the difference between getting all the gears and only getting some of them.
As a starting point, I suggest putting the car in neutral, loosen all the bolts, then give the lever unit a good shove back and forwards to make sure it really is free to slide and be adjusted. If there is any bend, for example, in the steel side panel of the central tunnel, this could hinder the free adjustment, so make sure first that the lever really is free to slide and be adjusted.
Once you're sure of that, then you can see the two extreme positions possible, furthest forward and furthest back. Then as a starter, put the lever in the mid-way position and tighten the bolts enough to hold the lever firmly. Try to see what gears you can select. If all of them, fine, tighten the bolts fully and job done. If not, slacken the bolts and start experimenting, moving the lever forward about 2mm and repeat. If no success going forwards, start in the mddle again and go backwards.
If after all this you still can't get any location where all gears can be selected, then it looks as if there might be something wrong with the gearbox itself.
