1. The traveller car comes out of the integral track at one end..what is supposed to stop that? The seller said he used a knot on the mainsheet but I think there should be a stop somewhere in the track.
There should be a stainless steel plate that covers the opening in the mainsheet track:
There are stops screwed into the track for the jib travelers.
2. I have bought 3 x swivel clam cleats for the main and jib but they have flat bases. Can I reuse the old ones and will self tappers do to secure them?
The old ones can be re-used. If you have removed them, they require a rivet in the hole that's not under the cleat, then two self-tappers in the middle two holes (they attach both the cleat and the base), and a machine screw/nut on the last hole (that's clear of the crossbeam).
3. There's a twist toggle on the end of one side stay but not the other...are they that important?
Yes. They are required equipment.
4. What diameter is the tiller rob?
Tiller "rob"? Not sure what you're asking here.
The stock fiberglass tiller has an OD of 1/2".