1. Yes with a small bit of cut and shut. Basic width and depth is close enough, same heater unit etc.
2. Both can be used. A lot of people for simplicity keep the KA box and shafts etc for simplicity as it solves g/box mount issue. However, puma set up is better with equal length shafts etc. However it wont bolt up dead easily. You need a custom engine mount anyway so knocking up a custom gearbox and torque link isnt really a big deal. Also you may need to cut and shut the wishbones or cut and shut a puma subframe to fit as the track is slightly different and ideally you want it all from one model to make it easier to fix and to keep geometry and ability to bolt together spares the same.
Had i continued with my ka project I was planning on fitting a puma subframe to the ka with some relatively simple shell and subframe tweeks.
3. Changing the whole loom is fairly easy like that although you will need to think about how you will fit the battery junction box in engine bay plus a few connectors need swapping over.