1) Depends on the motherboard.
2) PCI-E 2.0 has 1/3 the bandwidth of PCI-E 3.0, but generally modern graphics cards do not take full use of the bandwidth of 3.0
3) If there is a video capturing device in the third slot, nothing should happen to the bandwidth of the first card.
4) The full length is so you CAN put a GPU there if you so please.
5) No idea what DV is.