The reason is because there may be some problem with the ignition circuit. Bypassing the ignition switch, ballast resistor, etc. is a simple way to ensure that they are not the problem. It's only a test.
I'm afraid I don't understand your question about which terminal. I'm talking about a jumper from the "+" terminal on the battery (the one that is not grounded to the engine and/or body), to the "+" terminal on the coil (the one that does not connect to the distributor/points).
To verify if I am at TDC, I have an old compression gauge cable with no valve, a simple tube that I blow in, if it holds pressure thanthe vales are closed, if not than I am at the wrong tdc.
But that will only tell you if the valves are closed. I am talking about really finding the reference point where the piston is exactly at the top of the stroke. The point being that anything else you might look at (the valves, the timing mark on the front pulley, etc) may NOT be accurate.
It is actually a fairly common problem with the TR6 motor for the timing mark on the front pulley to be out of place. And since your engine has never run since it was assembled, I would be looking for a definite confirmation that the camshaft was timed to the crankshaft correctly. Even one tooth off can create an engine that is very hard to start and will not run properly. And the old marks are not always accurate (plus anyone can make a mistake).