If you can run the power, go .22.
You need to stay below 950fps for best accuracy, calibre irrelevant, otherwise you enter the transsonic range and you have problems.
So if you can get your gun to give you ~925 fps with a heavy .22 then there is no reason to go with .20 and plenty of reasons not to.
Your 3) - sorry, but little of that is correct, probably nothing at all...