Take heart, my friend. All is not lost!
Hairline cracks won’t necessarily prevent you from shooting if they really are hairlines and not deep structural defects, especially with the light loads you wisely intend to use.
The thimble appears to have been soldered on. Depending on how the rib is attached to the barrel, reattaching the thimble might be a simple soldering job, or it might be complicated. If the rib is screwed or riveted or (less likely) brazed to the barrel, you can clean off the contact points, flux well, and solder the thimble back on with low-temperature, high-strength silver solder. If the rib was soldered to the barrel, it might get more complicated, as resoldering the thimble may melt the solder bond between the rib and barrel. That might be a job for a professional. If you need help, there are very capable people here on the forum who have the skills, if you can talk them into doing it.
The nipple should be pulled. I have had good success soaking the breeches in straight kerosene for a few days. If the nipple won’t turn after that, try heating the bolster with a propane torch. Not too hot… there’s no need to heat anything to the point that it changes color. Just too hot to touch. If the nipple still won’t turn, let it cool, then reheat and try again. The heating/cooling seems to help with breaking the bond.
I have heard of the barrels of some percussion revolvers screwing into the frame with a left-hand thread, but I’m not aware of any nipples with left-hand threads.
The best way I know to check male threads is with a Ruelle Screw Checker. These are available in fractional or metric sizes. I know that Brownells, Lee Valley Tools, and Victor Machine Tools all sell these. I use mine frequently, and recommend them. Once you determine your nipple’s thread size, you can look for a replacement, although if the original nipple is still usable (or fixable), and it isn’t too loose in its seat after you have cleaned it, there is no reason not to use it. I would put a dab of anti-seize grease on it before putting it back. I would be very wary of trying to chase the female threads in the gun’s bolster with a tap. It you don’t get the tap started exactly right, you can booger the threads.
If you hit a wall with regard to the ramrod pipe, the nipple, and so forth, I would recommend contacting Mr. Robert Hoyt. He is best known on this forum for lining and reboring barrels, but he is a very skilled machinist and gunsmith and can help with all sorts of barrel issues, including soldering.
That is a really nice pistol, and I remember seeing it for sale. I even considered buying it. I’m glad it went to someone who appreciates it, and intends to shoot it! I understand that you found some issues with it, but from what I have read, things look fixable.
Please keep us up to date with your progress.
Best regards,
Notchy Bob