1) This will be dug up and cable laid underneath, and then tarmac or concrete will be relaid ontop
2)Cable will be/can be split in the service box on the outside of the property. Cables can go what ever way suits you.... to a point
3) Yes, the length does effect the signal but this will be compensated. For a typical install, im sure your house will be fine.
4) If I came round to your house and you wanted me to run them under the floor (since they are already exposed) I would be more than happy. The tech will be more than happy to do this. Dont expect them to lift or replace any mind!
Hopefully this has answered some of your questions