Is Espruino right for my project?

If I use the the custom espruino graphics canvas how will it work when running in a browser?

You'd have to write your own version of 'drawString' just for the browser - but you've basically done that all already.

Using my own (very simple) byte array canvas will work in the browser and on a micro controller

It will, yes - but chances are it'll run too slowly to get the 24fps update you want on Espruino - unless you run it on something like a Raspberry Pi, or you re-write everything in C.