Q. I have found with my meter that if I go from positive to negative terminal and tweak the balance if it reads something its usable and if not it's probably garbage if spring makes good contact on balance.
A. This is the best test I know of. It will show you if the circuit is complete and if it opens and closes.
Q. I also found some balance and cocks work and have less end play in some movements than others. I'm guess the white jewel is broken in the mainplate maybe.
A. There are several 'electric' models. and the balance assembly is not compatible across them in most cases. For example you can't use a Dynabeat on an Electric.
Q. How does one hold the movement when replacing the bridge as the calendar plate side secures the 3 screws.
Which movement are you referring to?