That was one of MS/Asobos very weird ideas to choose an high altitude airport for training, but with correct leaning this shouldn't be a problem.
Maybe in the realism settings auto mixture is already enabled and your plane performs already realistic.
If you pull the mixture knob towards you and there's no RPM change, auto mixture is enabled and you can forget about it.
I would recommend using auto-mixture in any case, since this is still more realistic than the broken leaning engine, plus you don't have to fiddle with the red mixture knob.
No keys for trimming! IRL the hat switch is used for trimming. If it's not assigned you should do that.
Push the hat forward/up to trim nose down and pull the hat back/down to trim the nose up.
That way you have to immediate feedback about the correlation between stick pressure and trim effect.
Note that trim response is a bit sluggish. You trim a bit nose up/down, wait a bit, continue trimming, wait a bit and so on, until you can let go of the joystick.