Thanx for hint, I put a link to DB2010 patching tutorial...

1. You loose all patches if you reflash the MAIN firmware. If you reflash the FS, then you will loose the user data (any settings, contact list, ringtones, ect.) from phone's internal memory.

2. If you want to apply the 'unlock' patch generated by my tool then no need to debrand, just use a proper MAIN firmware to generate the patch. However if you want to put other patches, then it is required to use a generic(unbranded) firmware. Also it is required to match the firmware version with the patch.
Here is an example: Open the *.vkp patch with Notepad, and in first line you will see something like ;W810 SW-R4EA031. This means that you need to have a W810 phone with R4EA031 generic firmware (W810_R4EA031_MAIN_GENERIC_HN_RED49.mbn) to be able to apply the patch.

SETool2 Lite v1.11 is needed for applying patches to DB2020 phones only. To apply patches to DB2010 phones you can use the v1.08 as well.

3. MAIN: W810_R4EA031_MAIN_GENERIC_HN_RED49.mbn (R4EA031 can be any version, but for this one there are the most of patches)
FS: It depends what language you want(I suppose that you use english) so basically you can use ANY FS, because you will get the English with any FS. E.g.: W810_R4EA031_FS_EUROPE_1_RED49 (instead of EUROPE_1 can be anything)
Custom pack: The custom pack must patch with the FS: CDA102555/10 (EUROPE_1)
Here is the CDA List to find the required custom pack.

Just look at download section, and you will see what firmwares, custom packs exists for your phone...

4. Good question, don't know. Maybe someone will help you with this, sorry...

5. It is always a good practice to backup the GDFS and put it to a safe place e.g. a CD/DVD disc, USB thumb/flash/pen drive (choose the name you like :) ) etc.
If you brick your phone by damaging the GDFS, then you can restore the backup, so it will bring back your dead phone to life.

6. That patch is good for nothing, because like you said, the headphone is the FM antenna, and without it neither of radio stations will work ...