For starters, I would not try and time an ADR with MNSSHP. You'll hear stories of people trying to linger in the parks, but we've done MNSSHP and MVMCP, and CMs were out in full force checking for bands. I believe ADRs 6:45pm or later on party nights can only be held by people with tickets for the party. Every now and then you'll hear stories that a CM let them dine and then escorted them out of the park afterwards, but more often than not, you hear of people being turned away from their ADR since they don't have party tickets. I've heard that club level planning will even warn people of this when making ADRs through them.
As for figuring out the party nights...in addition to looking for the nights when MK closes at 7pm...you can also look at last year's party schedule as the days of the week for the party seem to map over each year. I did this last year and it was spot on. There is a rumor that the parties will be starting up earlier this year since F&W is starting earlier, but I don't think that's been confirmed as of yet.