Why presume the “us” means Catholics?
Seriously, I do think Our Lord is cautioning against turf battles or getting the idea that we can somehow “possess” Him. The unknown exorcist was not one of their group, they didn’t even know who he was, and yet he had faith in the Power of the Name and that was enabling him to do the Lord’s work.
Compare Matthew 12:30 and Luke 11:23, where the reference is to anyone who is “against me” (meaning the Lord, obviously) and was directed at the “religiously correct” who were opposing His work.
To the extent that our non-Catholic brothers and sisters proclaim the Gospel and do works of mercy, we should honor them, I think. If they also revile us and persecute us and say all manner of evil things against us, falsely, for His sake – well then we are blessed, are we not?
From a practical standpoint, we are more likely to be persuasive if we can state the facts calmly and courteously in the face of a nasty attack then if we retaliate in kind. We may not convince the attacker, but others who observe the interaction may be moved by our serenity. So no, I don’t think you should smile sweetly and say nothing, but I do think you (we) should ask for the grace to respond truthfully and graciously (
gracefully).
As for what Protestants do with this passage, they do just as you do - they presume that “us” refers to the body of believers of which they are a part - however large or small they perceive that to be.