1) Basically you can use any modem for the I9100 (this includes the I9100P)
2) You can't back up a modem, or not easily at any rate. And you don't need to given how easy they are to flash. If you want to go back to a previous modem, simply re-flash it. If you don't have a copy of your current modem, you might want to find a copy of it somewhere before you go flashing something else (not sure why that would be important to you given your current modem is giving you no connectivity, but, anyways...).
3) CWM flashable modems are flashable in the same manner as any other mod/kernel/rom in CWM. You should already know how to flash stuff in CWM if you're running a custom rom, if you don't, you need to start doing some searching (and how did you get a custom rom on your phone if you didn't know how to flash with it ?).
Right. I've given you more than enough info & won't be posting further to this thread, this stuff is really very simple, and we have guides/tutorials (most of which are stickied near the top of the General section) you can read if you need further info, and everything you've asked can easily be found with a search.
Up to you now...