Marc,
Sorry for the delay in getting back to you today, I have been quite busy trying to get a job completed.
I will take your easy questions first
You said "Can it be that the "extra" cable (Red 8 pin plug on both sides) can be redundant in my setup?"
Well, the short answer is yes. The long answer is, this was almost certainly one of the cables already connected to the DVB-T module when you unpacked it. You will almost certainly have had to remove it and use the cable marked 03P456 at one end and 14P006 at the other (Wide plug)
You said "Is it correct to plug the longest coming from the module onto the small pcb thne the one new one (Coming from the back) onto that pcb as well"
The short answer is yes. The drawing in the instructions is very poor, but this is what they are trying to show in the very last diagram on the right hand side of the last page.
If you have further problems I suggest you provide a photo of any further areas where you are unsure. Anyway, see how you get on and let me know of your progress