It won't run the team selection screen in Team DM. It only shows it in OpFor CTF (Removed from Steam, at the moment, without a 3rd party install) afaik.
Go to options - multiplayer and you can select a model for your character. Changing this model changes your team, so if I select the 'Tower' model then I'm on team 'Tower' with whoever has a Tower model as their player model.
MoTD cannot be changed for individual maps, afaik, and if it can, it would probably be done with AMX.
That overwrite thing is toggled with the 'Insert' key.
And yes, why can't you download? Something like NetNanny I'd assume? (It's fairly easy to kill, but I won't tell you more than that...)