Couple of questions:

1) How can I bypass a 500k pot (tone or volume), so that the effect is the same as when the pot is on maximum volume/brightest tone? (maybe use a resistor that would have the same effect but wouldn't be variable).

2) Is it possible to use an on-on switch to change between 2 humbuckers? How would I wire them then?

Dear God, do you actually answer prayers?

Yes, but only in a way indistinguishable from random luck or the result of your own efforts.
1) A resistor wouldn't work because well they .... resist
2) Yes you can, there's tons of wiring diagrams online, google it. Id split the humbuckers with 4 switches then you can get a multitude of combinations.