1. If you reflash the MAIN file then you will loose the patches. However if you flash the breakloader, customize your phone with FAR + SEFP and at the end flash the rest(restore) file instead of the MAIN, then your patches will stay. So again, you loose your patches only if you reflash the ENTIRE MAIN firmware.

Or you can use XS++ v3.x to customize your phone and you will not loose the patches.

2. Probably yes, but I haven't tried it.

3. You can backup and restore your data with MyPhoneExplorer. This software will not backup the MMS/WAP/Internet settings(profiles), but you can backup them manually: download all files from /system/wap/profile/ (phone memory)

4. The trick is, that every GDFS is unique, so you cannot just flash another GDFS, because your phone will not work. But if you have a GDFS backup, and your phone's GDFS will be damaged, you can restore that backup, so you can unbrick the phone.
If you have a backup, then no need to worry, you cannot kill your phone :) .

5. The original patch is not mine, IronMaster is who created, and my tool ports it.
Every service provider has a code what identify it, and it is stored in SIM card. In protected area of GDFS is stored what SIM cards are allowed. Yes, the check is in MAIN firmware. In normal conditions(without the unlock patch) the phone reads the SIM-lock codes from GDFS and compare them with the codes read from SIM card, and you know what is the result :D
The patch will deactivate the mechanism, and give an "unlocked" code to phone, so it thinks that the SIM is allowed to use. The whole thing is a bit more complicated, but this is the main point.