Not without taking the phone apart, which is probably beyond the capabilities and/or motivation possessed by most people.

This was possible with my old Galaxy S Relay, for instance, because all the keycaps were the same size and shape except for the space bar and a couple of the modifiers. Once you got the keyboard separated from the bezel you could fiddle around with it as you saw fit. But not, just to name one example, with my Blackberry Priv because the keyboard had that godawful capacitive touch capability and the keycaps were bonded to the switchboard somehow.

No, that would be awesome if you could though.

I had the idea of using some stickers for mine if I really want the hardware to match the software.

Yeah, but I bought the phone knowing the layout. I wouldn't have bought it if it wasn't right for me, so not being able to customize it later isn't a problem.

Physically no, but you could use something like Pastiera to remap them in software.

I'm not sure what you mean by layers. Like symbols? And I have, and it's okay, but I never got good enough at it for it to feel natural. I mostly tap type with 2 thumbs, or 1 if it's quick and I'm lazy.