If you just want to reread something, it's quite unlikely that you'll want to do this while under heavy stress, ie. a situation when you need to scroll fast.
"I need reinforcements at Alba!"
"I'm sending troops to Ventris, but Corianas is on its own!"
So imho you probably only need the message box while you have some time on your hand. Fast communication with your ally in multiplayer will anyway be done by teamspeak or something alike.
I'm sure thats the theory, but with VOIP not built into the game...
You also could remap this key to something else of course, like the q key or so.
"Ok, I'm ready to..." "Ger, I can't type in..." "Gah, my cue buttons borked!

)

(I know you didn't mean that q should replace enter, but its funny

)
Why should this prevent you from scrolling?
When you write a message, you'll anyway have your hands in a position to press enter.
It makes it harder to scroll because you have to press the enter key before you can scroll, and that makes for a klunky (AKA bad) interface.