Finn wouldn’t have saved his friends by running into the cannon, wasn’t that the whole point?
I’m not sure. It wasn’t made clear in the movie, which was probably not a good idea.
The weapons clearly melted from the beam but it’s unclear of the crash damage would have worked.
Don’t forget that Poe tells Finn that the cannon is charged and that it’s a suicide run.
It being a suicide run doesn’t mean Finn sacrificing himself wouldn’t have disabled or destroyed the cannon, Independence Day style.