Sentinelese,
You are trying to compare some of the most isolated people on Earth to the rest of us?
Do they not eat, drink, make babies and sleep?
I live in Canada and there was no mention of this on any news, possibly the source is confused by wording, not asking King Charles to declare sovereignty but asking for a cancellation of state visit to remind America that Canada is a sovereign nation (has been since 1867), and not unoccupied territory suitable for annexation.
Now, as to what we would do if China invaded, NATO and NORAD treaties obligate all members to assist any member facing a military invasion, so China would probably know this and only invade if they wanted to take on all of NATO. This is why Zelensky wants NATO membership, he thinks that plus a peace deal would keep Russia out of his territory in the future.
King Charles III has no constitutional role in Canada. There is a governor general here and in several other Commonwealth countries representing the Crown, but in reality, “the Crown” just means “the federal state” and in practice the Governor General is a figurehead with no role in government at all. In theory there are various things they could do, for example, they could dissolve parliament if they believed it to be necessary to prevent treason or obvious harm, but other than one instance in 1926, no governor general has done any such thing.
King Charles III plays the same role in the u.k. and it has also been a long time since any British monarch played any important role in British politics, other than a symbolic role.
The G-7 meeting will be in Alberta and in a mountain resort (Kananaskis) so the potential for demonstrations will be less than if it were in a big city. There will be some anti-Trump signs, possibly anti-G-7 also, and chanting by stoned leftists, but don’t assume it represents a very large fraction of Canadians, most people here can understand what motivates Trump and wish we were taking some of the same steps too. Perhaps we will be after our election later this year.