before I order all willy nilly thread sizes battery bolt down and rear seat

They're typically coarse thread. M6 is very common (10 mm socket). You need to consider length too.

 

The small machine screws that thread into the bottom of the modules are the same thread as the battery module terminals M5 x 0.8

The length of thread under the head is 8mm, of which 0.8mm is an intergral washer, so really 7.2 mm is available for threading. The head size is 8mm hex.

And the small bolts used in the case are M6 x1.0 with about 8.2mm of thread under the washer. Head is 10mm hex.

If you broke one or two off in the modules, you could just leave it be. The way the modules are interlocked together, it won't make any significant difference.

If you decide you want to remove a broken bolt from the module, the best way to do that (assuming you can't just grab it and rotate it) is to use a dremel tool with the very thin cutting wheel. Use it to cut a small slot across the broken piece, then use a small flat head screw driver to unthread it. It won't matter if you cut into the brass bushing a little. If you're looking to drill/tap, I would recommend not. Much more likely to damage the module.

 

Great, thanks!