The situation was an enemy scurrg engaged with howlrunner and rhymer.

What I defend without much conviction but attending the words. As long as it was engaged with two squadrons (heavy prevents the effects, not the engagement) it couldn't attack my ship. Am I right?

Yes, but Not for that reason

Grit only allows you to Move if you are Engaged with only one Squadron. It does not effect Shooting.

In this case as well, you are still Engaged by 2 Squadrons, however, Heavy does not stop you from Moving or Shooting. But its still a risk of shooting at you, which gets your attention somewhat, which is what Engagement partly represents.

From moving... so it couldn't shoot even engaged only with howlrunner. It means that the previous shots on my ISD were ilegal. Anyway I won so I am not gonna care about of it.

Thanks!