Welcome to this forum.

My opinion regarding question 2: if you have a good quality digital mixer with built-in effects I suspect you won't get improvements using the keyboard mic input.

I get a better vocal sound using my digital mixer than using my Ketron module mic input. Less noise, more variety of effects. But for live use the difference is not noticeable, so I end up using the keyboard mic input and leaving the mixer at home.

-- José.