Changing a phone extension at master socket

[Fazerider]

Q1: Yes. No, a very short unterminated length won't cause any problems, but you can wriggle the unwanted wires out from the connector with a pair of pliers.
Q2. Yes. Don't do it in a thunderstorm, otherwise it's harmless enough. The 50v dc line power won't give you more than a slight tingle, but if someone calls you while you've got your fingers across the wires the 75v ac might make you jump.