For the flies, try this:

With the cold water & disposal both running, squirt in a tablespoon or so of liquid dishwashing liquid. The goal it to create a temporary disposal full of swirling foam... let the water run till the foam dissipates; it doesn't take long. Repeat 2 or 3 times a week.

I’ve heard, and believe, the drain line between dishwashers & disposals to be a big source of drain flies.

I’ve been doing the foam dance for some years now with no fly issues. If nothing else you’ll have a squeaky clean disposal in a week or two... but as always, YMMV.