What if an authoritarian party takes overs United States and attacks Canada, but Canada invokes article 5 against another member state the USA?

This is possibly the Achilles Heel for NATO, being a member attacks another member.

If NATO fails to defend said member who was originally attacked then NATO is politically weak proving their alliance to Russia & China that they are indecisive when it comes to internal attacks from other members.

An authoritarian American government will not listen to diplomacy but I'm assuming that's the first thing that would be done.

After failed diplomacy, NATO must prove itself by directly coming into the defense of Canada.

In this hypothetical I can see them doing the following...

Asymmetrical submersible warfare such as cutting American sea cables and shooting down satellites and providing weapons to Canada via the arctic ocean.

They can also deploy special force teams to engage in guerilla warfare and use Ukranian drone tactics to blow up American equipment.

Edit: Don't underestimate smaller navies, when used correctly they can still inflict heavy damage.​

Would NATO prove indecisive or would they go to a proxy/semi-direct war against the United States?