One Keypad can only be paired to one Nuki actuator. You would need 2 separate Keypads for your use case.
However in theory it should also work with one Keypad if you connect everything via IFTTT (i.e. create a rule that unlocks the Smart Lock when the Opener has been unlocked by a specific User/Keypad Code), but this might cause a too long delay between the two Openings.
Yes, should work. Follow the “Other door” scenario in this guide: Howto connect the Nuki Opener to unknown intercom systems