While I was removing the black rear seats, I noticed this bracket is missing the bolts.
What size bolts are these? So I can buy replacements.
M6x20 with a repair washer
What is this red line for?
Central locking vacuum pipework
This yellow connector is different from the back-rest to the car's connector. So I can't connect them together.
I think you are missing the outer shell of the connector which has the half-round shape and the locking tab. Will that piece actually fit in the car-side connector?
I removed the black door cards and I need to know if these are still available?
4D0867299D, still available from Tradition
While I was cleaning up the oxblood door cards, I noticed one of them has a very bad tear in it 
.
Can this be repaired?
To an extent athough likely not invisibly. It looks like the plastic panel has broken, so that'll need bonding / welding back together from behind so it pulls the vinyl back together. Once thats all sound the vinyl can be repaired with a filler compound. It'll be visible up close, but probably good from ten feet.
Can this part of the leather be fixed? Or do I need to take it to have it professionally repaired?
Because of the extreme difficulty of getting this apart without destroying everything, my stock answer (at this time) is no. The leather, foam, and plastic backing are all plastic-welded to the main door card, so to get them apart you need to mechanically separate the weld, without ruining the leather. You can't get the leather out from the front, and I think it would be practically impossible to get it back in and look good if you did get it out.
I say 'at this time' because I haven't done it yet, and I haven't seen anyone else do it either, however I do have precedent for cutting up and customising door cards so it is on my list to try. If I figure out a non-destructive repair then I'll post it up.