Jet2 fly to Geneva throughout the summer months so you dont need to use MAN
Indeed they do but a single flight on a Saturday isn't much use for a Monday-Friday commute
As for security, it makes no difference, airside or landside, it has to be gone through again.
But when arriving from Manchester into T5 I don't have to go through security at all - a quick boarding card check at the bottom of the escalators and you're in the departure lounge in less than 5 mins from doors open