"Intake manifold blew"
Why?
Intake manifolds are set up to hold negative pressure, vacuum
An actual "back fire" coming in from an intake valve can "blow" intake manifold gasket, and this would indicate a cylinder with an issue, so intake gasket issue may be a symptom
As with all misfires doing a compression test first is best, just to take compression issue off the table or find out it is the issue so you don't waste time and money on spark or fuel issues
If compression is equal across all 6 cylinders then a leaking injector could have caused back fire and now a misfire
That should have been seen when changing spark plugs, one of the plugs would have been much darker than the others, indicating a richer mix
Your MPG should have shown signs of dropping ahead of time
I have been wrenching on vehicles for 50+ years now, and I STILL miswire firing orders, and I KNOW THEY ARE CORRECT, until I find out they ARE NOT
5, 6, 4, side of coil pack can be wrong, at the coil pack or the drivers side cylinders