Buildings.... Hmmn...

They are hand drawn, and have a lovely personal style that doesn't seem to be struggling to imitate something else. I wouldn't change them (unless you really hate them or something).

Maybe, since you have made the roads and the trees and grass relatively dark and colourful you might consider a touch of colour on the rooftops? Not too much, though. Not too dark. At the moment they stand out clearly because there is that light/dark contrast, but maybe just very slightly reduce that contrast by adding just a little shading and colour - a bit like a tidy watercolour, perhaps?

I don't know - and others will probably have different ideas. Maybe you will suddenly have an idea of your own about how you want them to look

Numbering on a city map is something I've never done, but I assume its like labelling. The numbers would have to be close to the thing they relate to so that there is no ambiguity about which building or area they are metaphorically attached to, but placed in such a way as to not get in the way of the general view. It can be a tricky thing to do, and you may need to give each number a slight halo of white so that it doesn't get all tangled up with the line work of the map.