The mast will rotate to 90 degrees once there is load on the sail. Even with tight sidestays the mast will rotate and push its way into the stays.

Its almost impossible to get the rig tight enough so that the leeward sidestay stays tight upwind when everything is cranked on hard. Same rule applies with the kite up. Trust me, when you've got enough wind to fly a hull, you should have enough breeze to be able to rotate the mast to 90 degrees.

The wind pushing into the mainsail will be enough to rotate the mast. If not, just run in and give it a bit of a shove but the more wind, the easier it is, and the more likely it is to stay there.

With the kite flying, if there is plenty of breeze the mast should rotate and stay there, though sometimes if the loads come on at a different time out of a gybe (ie, kite loads up before mainsail) the mast might not rotate all the way around and only go to 70-80 degrees (you'll notice the difference if you look up!).
In really light breezes <5kts sometimes my mast won't rotate and stay there but you're not likely to break it in that stuff anyway.