The sill plate touching my foundation isn't laying flat, it's about 10 degrees tilted. How do I install a header below it?
Ideally, you want the plate to fully bear on the header. The fact that a twist developed indicates there is not any appreciable load at this location, so it probably does not matter much what you do. It's tempting to try to take the twist out so everything is straight and true, but you may just end up causing the studs above to not bear on the plate. The installation may look better, but you have not gained better bearing. Further more, the suspended studs could eventually settle, causing cracks in the finish and creating stress risers nearby due to differential settlement.

This leaves the viable choices to either shim or cut the header top on an angle to match. While shimming has the appearance of shoddy work, it is actually the best way to ensure full bearing. Focus on getting good bearing under the studs above, the space between studs is not that important.

You should place sill sealer under the window's sill plate, some standing water could potentially sit here. You don't need it behind vertical members to protect the PT wood from moisture, but you will want to do something to limit infiltration. Sill sealer works well for this, but you can use other methods if you prefer, such as polyurethane foam, or simply setting the king studs in sealant.