Are Jet2 going to be using the main satellite building (gates 1-19)?
Yes, Jet2 will be operating from Satellite 1 (Gates 1-19)
Does Sat 1 has enough stands for EZY, TOM, MT and LS for the morning peak?
Not quite but since the Jet2's generally depart a bit later, they'll probably tow any aircraft on remote stands over once the first few aircraft have left.
Stansted is very 'peaky' - huge demand early morning for departures and arrivals late in the evening
Last year they opened the new gates 90-93 which are located in a coaching facility below the Terminal used to access remote parking in the morning rush.
Would MAG perhaps prefer to wait a few years so that airlines can be forced to schedule more flights in the currently vacant non-peak slots first ?
Not sure that's a particularly wise way to manage growth. This is already being accounted for largely by Jet2 who mostly will be operating their second wave of departures between the 14:00 - 16:00 period which has often been much less busy.