1. Should be fine, if worried about UV breakdown over the years some PVC cable capping over the top will protect it from UV and rain etc
2. You can place the openreach modem at the master socket, and then run up to 100m of proper Ethernet to the router if you want OR when ordering ask for the Home Wiring Solution (HWS - which is free) and a data extension will be run for up to 30m from the master socket. Colour - not sure, white I believe.
3. You can never have enough Cat5e/6 installed, but most people do two or four to faceplates, and if need more just add an Ethernet switch on one port.
4. Ethernet switches need power. But for the passive Ethernet cables and patch panels they are unpowered.
5. Cable length for Ethernet is not really an issue, unless looking at runs over 100 metres