I imagine that the lack of a DevTools front-end and extensions on mobile are product decisions, not technical ones. There's probably a compiler flag you could flip to compile an APK with support for both. Mobile devices have historically been small and underpowered; Google may have decided that including extensions would expose too much surface area to slow the browser down and/or kill the battery on a mobile processor. Similarly, they must have decided that the DevTools UI was designed for computers and wouldn't make sense on a small touch screen.

I'll be very surprised if they remove either feature from future Chromebooks.