I'm pretty sure I understand this, but I want to be sure. If a squadron possessing Grit is engaged by two squadrons, one of which has Heavy , it's still as stuck as if it were against two normal squadrons? Example, a YV-666 is attacked by an X-wing and a Y-wing. The YV-666 has Grit , but still is stuck. Am I right?

Yes, and it is standing in the FAQ in the same way.

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Q: Is a squadron with grit prevented from moving while engaged with two squadrons, one with heavy and one without heavy?
A: Yes