Unbranding my W850i

[GamerMat]

- Do I have to replace the firmware with R1ED001 (as I have now) or can I use the newest R1JG001_RED52 ?

You can use any firmware, but that's if you unbranded it using DaVinci, or any other similar things.

- What FS should I choose between: CENT_EUROPE, EUROPE_1, EUROPE_2, EUROPE_3, EUROPE_4 or EUROPE_5 ? (I live in Belgium)

I really don't know about this, but you could choose whatever it contains your language.

- I like Vodafone's Orbit theme flash menu But I know it will be gone after debranding. So is there a way to restore it after the process?

You can restore it by flashing the swf file of orbit to the phone's firmware file.