Is "de mult" obligatory?
No it's not obligatory, but it is one of the many forms that we add to a sentence to emphasize.
Does it give it a meaning more like "You eat more than enough"?
I tried that, but it's marked wrong.
That would be "Mănânci mai mult decât suficient' (I think)
So, again, you have to put "mult". "Mai decât suficient" doesn't work...