Note that if you change the message type, the default AI-generated message will be written again, and overwrite your changes.
What I do is to select all the text and hit CTRL-C to copy, then paste it back whenever I need to.
This also lets me close the dialog window to look up the name of the System I'm talking about, or to double check coordinates or anything I need to. When I go back in, I hit paste, and keep going.
The tone you set does alter the perception of the message. If I remember correctly, use pleading if their score is 20% above you, demanding if it's 20% below you, and neutral for the middle.