Ok, I'll make some platform to raise the car by the wheels and work under it "comfortably". Maybe I'll drive to my workplace which is a truck workshop and we have pits there. It is a 10 km route.
A) I will able to select all gears with little effort. What's next ?
B) I will not able to select 1st/2nd. Gearbox remove and inspect ?
Last weekend I removed the starter motor and with a steel ruler I reached the primary gear which I was able to rotate while the pedal was depressed.
A and B - yes. The reason for doing this test is to establish with some good certainty if the problem is Internal or External. It's a lot of work to pull the Engine, take the Gearbox off, then Strip the Gearbox pretty much to the last nut, if only to find the issue was in the Shifter,
And I won : the O-ring at the detent sleeve is missing - because I did not put it in of course 
Heheheheehe,,,,,, personally, I wouldn't call that a 'win' 
I have Helicoiled the Diff Side Plate Threads in-car, so if the Engine doesn't need to come out, I wouldn't be pulling it out to do this job. I did have to use a long drill extension to get to some of them, though if I had an angled head drill, I could get to them with that,,,,, reminds me,,,, the missus has one,,,,maybe not for long,,,,,,