The NATO Pact, established in 1949, is a collective defense agreement where an attack on one member is considered an attack on all.

Even an attack by one of its own members that's its leading military and financial force?

Yes, in fact that would be the most important time to follow through with that agreement from the position of the other members.

Yes

Collective defense aka casus foederis as defined by Article 5 of the North Atlantic Treaty.

Even an attack by one of its own members that's its leading military and financial force?

Absolutely. Yes.