And can I use both my spell and weapon in the combat check as long as they both only take 1 hand?
Go for it.
Can I change my weapons/spells any time during my overall turn before combat check starts?
Yes, but remember that once you quit alloting hand(s) to your spell(s), their effects end and also, since they are now exhausted, now recasting them until you've unexhausted them during the next Upkeep.
Or if I fail a pre-evade pre-combat spell can I switch to a 2 hand weapon for combat?
Pre-Evade ? Combat has 2 phases, Horror Check and Fight or Flee. If you fail a spell check for a spell, it will still take up the hand(s) for that round of combat. On the 2nd round you can change to another weapon.