Another question though if we wanted to go just for MNSSHP and enter the park at 4pm would that use a day of our Premium ticket??
this is a separate ticket
so, it depends what you do that day.
If you have a 7 day premium, IF you go to another park that day (or even MK) before 4pm, then you'll use one day on your ticket.
If you don't go to any park that day, come to MK at 4pm and use your party ticket (do not put your premium ticket or key to the world in the turnstiles, only your party ticket) then you don't use a day on your premium tickets
if you have 14 days or 21 days it's irrelevant, since those expire at 14 or 21 days not depending how many times they've been used.
A 14 days unlimited (which is only a wee bit more expensive than a 7 days premium) would allow you to enter a park in the morning at no extra cost, whereas the 7 day premium would use one day for just a few hours
alternately on a 7 days premium you could use a waterpark or disneyquest entitlement in the morning which wouldn't use a "day" as they are minor parks.
If you don't have your disney tickets yet, have a look at the 14 days ultimate rather than the 7 days premium, 14 days are only £7pp more than the 7 days premium ... that would solve your issue
ETA : Dixonsontour's solution (MYW US tickets) works if you don't care about waterparks or hopper. If you wish to have those options, then go for the 14 days UK, if you don't need them, go for the US 10 days