The mainshaft is in the engine and in the 4th pic you posted in your first post, there is an intermediate shaft that goes UNDER the front cover, and on the end of the main shaft. That intermediate shaft then pokes out of the front of the front cover, and the driveshaft slides onto it, and into the front diff.
You have to unbolt the front diff and slide it forward to get the driveshaft in, but first you're going to have to remove the front engine cover and install the intermediate shaft.
Of course, the big question is, why was this taken out in the first place? I'm guessing something is stripped or broken somewhere in that mix of parts. Check the splines in the shafts and see if there is play when you put them together. It's hard to see the front of the main shaft inside the motor, but from what I can see of the pic it doesn't look all that great.
So in that schematic, #2 is inside the motor, #1 slides on the end of #2, but goes BEHIND the front engine cover, and it is missing (thus the hole), but appears to be in the 4th pic in your first post, right in the middle.