It doesn’t look like any of those will work on 12V, and even the 2903 I’m not so sure if it is a good idea.
If you get the panel running and need NA’s for it, I would recommend trying to pick up a Wheelock MT 12/24. You can find them pretty cheap and they are pretty much 8 alarms in one, as well as operating both on the 12V you need now and the 24V you may upgrade to in the future.
Kinda sliding off topic here but I will answer your question, the third jumper on the SHG is not used and serves no purpose whatsoever. I like to use it to store the 4th jumper plug when I have it removed (to set the unit to march-time code).
Unfortunately, I think you are in a deadlock, so to speak. You will need to get a 5230 to program the panel, and even then there could be damage that shows up which we can only speculate on now. Keep checking eBay, someone is bound to throw one out there that’s beat-up and cheap, either that or they don’t know the potential “value” of what they’re selling and just throw it out there. In the meantime, you could consider using the money you would spend on the 5230 on a more suitable control unit. You can likely find a replacement unit cheaper than what it would be to acquire a 5230 to get the old unit running.