I agree, the message window should be floating.
It's not too much to copy/paste the co-ords of a point (poi, player, tunnel or camp/outpost/base) but getting a players name right is definitely a problem as some have difficult combinations of letters and numbers and long names too.
If multiple players are all part of my alliance then it is simpler to send to all alliance and delete those you don't need to send to (although this doesn't happen much).
Creating a special group similar to veterans etc where you could mail them as a group could be useful where you have dig teams and wish to address only dig team B.