Have I damaged my tyres?
Those strings are quite common with continental tires and seem to be a part of their manufacturing process. Every continental tire I have owned seem to have some of those threads, even ones that mounted relatively easily. I just cut them off and have never had an issue with them. Continental tires are also well known for their tight beads.

The high-pressure technique is a common technique to get a bead to seat. To speed it up you can also put the tire in a vice and pull on the top of the wheel to try and force the bead to seat.

As an aside, many recommend against running fenders with  knobby tires, as the knobs can pick up debris (e.g., small sticks) that can jam in the fenders causing a crash.