Yeah, in aftertime I've come to the conclusion that it must've been the FS that was broken and that I should've just flashed that part as well.
But I was kinda desperate and I sort of couldn't explain to everyone that it might've been my fault.
Either way, the firmware is back to K750i R1DB001 and the FS was never actually flashed to a W800i due to an error. Whatever they might find not suited for my phone (W800i content) will be erased when they flash the FS as well. So no traces left of my modding (hopefully).
I really hope they don't find anything, cause it said somewhere in the contract I signed that they can charge me for about 75$ if they find out it's my fault it's broken.