The Ls must be connected together,
the Ns must be connected together and
the Es must be connected together.
Any suitable connector may be used. The Es can go in the back box E terminal.
Yes, your diagram is correct.
It has recently been made against the regulations to use the earth wire as a live conductor but it was not when installed.
It would be good to sleeve it brown but it doesn't really make much difference.
Yes, you can have one with 2 switches and one with 1 switch; you can have one with six switches if you want.
You are comparing different things, the two-way switches are needed to work the same light from different floors but there is only one switch on each floor doing that. Any additional...