1) Original Amadeus 16.25" x 19" plinth
2) Have no idea. Someone here at AC might though.
3) Not even remotely to be considered. The Amadeus is excellent integrated design with an already superb tonearm. IOW, the synergy you're looking for has already been taken care of. Swapping arms would be a fool's errand and near impossible anyway. Plus there's really no room on the base/plinth to mount a second arm without basically destroying the table.(something that might also occur in a fruitless arm swap attempt) To put it simply, this is definitely NOT a turntable that's been designed for arm swapping or a second arm. The only possibility "might" be to mount a second arm on an external arm pillar. If you're dead set on using a second arm, consider a second table.
4) Buy it new with dealer support or used from someone you know and trust.