1) The way I always tell people to approach this is similar to what I said to the OP; go to Samfirmware, find all the stock roms released by your carrier, click through on each one & make a note of the modem. Search here/Google for CWM flashable versions of each modem, and test them out. If you find you go through all the ones bundled with stock roms released by your carrier & your connectivity is still bad, do the same with modems bundled with stock roms released by other carriers in your country. If your connectivity is still bad after that, flash any modem you can get your hands on.
But for most people, modems bundled with stock roms released by their carrier should work best; rationale being you'd hope the carrier would have fully tested them to ensure they work for most people most of the time on their network.
If your connectivity is currently fine, there's no need to go messing around with modems. Chances are you'll make it bad if you do I.E if it ain't broke don't fix it. Only mess around with modems if you have problems with connectivity.
2) Yes, modems are independent of Android version.Same risk as flashing anything else (I.E very small risk of the flash going bad with any given flash).
3) Risk should be minimal; I've never heard of anyone borking a phone with a modem misflash, but anything is possible I guess.
If it's important enough to cause you grief if it's lost, back it up. No excuses
Trolling for thanks is lame
Does Samsung's auto-brightness implementation crap drive you to self-harm ? A solution !
Failure to search is the leading cause of male impotence on XDA
If you don't know how to recover if things go wrong, what are you doing flashing custom roms ?