1.When going to a location with two monsters and you fight one can you still sneak by the other? Or does fighting ONE alert the other?
You can fight the first monster, and evade the second.
2. When a monster moves in your location, you can adjust your sliders before the evade or battle correct?
Monster movement typically happens in Mythos phase. You don't have to fight until movement phase. Upkeep is in between, so yes.
3. When rolling to lose items from Ithaqua can you use clue tokens to re-roll?
It is not a skill check, so no. You can only spend clues on skill checks.