Some people use a G-clamp holding on to the inside faces of the two pickup routs and suspend the guitar from that. It is still a bit of a balancing act. I just did one side of the body at a time on mine. A bit slower, but easier.

If you build a wiring harness using stiff wire with no excess cable lengths, you can put the electrics inside the guitar using just your fingers and some needle-nose pliers. I've done it. Some cotton around the toggle switch helps but it's not really necessary.

The neck really is simple as you can't go wrong with position.

I'd smooth the bottom of the pocket out a bit, but not too much. The small 'wings' on the top of the neck cut-out by the fretboard will hold the bottom of the neck off the bottom off the pocket if you aren't careful are remove too much wood just in order to get it smooth. Which is why you need a few good clamps to hold everything together when gluing it in, and leave it for the full 24 hours before removing them as the neck joint will be under tension.

If in doubt and there is a bit of a gap, you can stick a thin piece of maple veneer in the bottom of the pocket first and sand that back a bit if necessary. But make sure that gets the full 24-hour clamp treatment as well.

I've just glued the neck in on my ES-1 and it's fine.