A, Doesn't matter for hard wiring. Stranded is better for connection from PC to router etc.
B. No, not for the sort of speeds provided by ADSL
C. Best if the plugs match the sockets - as long as there are enough connectors and wired up correctly (!) it doesn't really matter.
D. Yes, they are best for inserting wires into the socket connectors.
E. Don't think so, except, as you mentioned, the inherent risks of fiddling with the BT side of things!