First of all I would like to point out to readers, that King Charles III is King of all England and Dominions beyond the Seas. So King Charles is King of Canada, Australia and New Zealand.
Mark Carney is currently in France and will have an audience with King Charles tomorrow.
I think the reason that King Charles has invited Trump for a second state visit, is to get him in a room alone and "read him the Riot Act".
This meeting will be behind closed doors, and what will be said will be confidential. Only the King will make the record, and it will not be published for a hundred years at least.
One or two of the King's Guard will be stationed outside the room. So Trump will have no escape. The Guard will certainly have a variety of Claive in their hands.
That is known as "showing the cold steel'. As the old British Tommy's used to say: - "They don't like it up em!"
So if Trump were ever stupid enough to try a military incursion into Canada, he would be facing troops from the Dominions and likely Nepalese Gurkhas as well. They have trouble staying out of fights like that.