1. not sure
2. see 1.
3. are you going to be doing the work yourself, or are you gonna get a mechanic to do it? As far as I know the gearbox will bolt up, the engine mounts should bolt up (which one doesn't?) the hardest part would be the wiring, which isn't hard if you've got a couple of wiring diagrams and a brain.
Externally the blocks are the same. The differences between CA20E and CA18DET are an oil fitting for the turbo and the deck height is a bit taller on the '20. All mounting bolt holes, etc are all the same on both blocks.
RWD vs FWD CA18 - AFAIK the only difference other than the wiring loom is the water pump feed pipe, it's a lot shorter on the RWD, and faces the opposite way. But you can swap this block-to-block anyway.
Use all your '20 ancillaries and mounts, and just bolt their brackets to the '18 block. Belts, lines and stuff should all line up.