10K pots are fine.
I bought some 10k slidepots (cheap) from Germany from www.pollin.de, it was a couple of yeras ago so i don't know if they still have them.
I think that Arduino can scan the 4051's so fast that there wil still be time to read the switches as well. Otherwise you could use an interupt for one or two very important switches, to give them "priority" over other controls.
The bigg issue will be the velocity sensitive pads 