I’m already thinking of my trip next year for MNSSHP. We will have tix for every day of parks. So entering before 4 won’t be a problem for me on that’s specific day( not to early don’t want kids to get worn out.) I have a feeling my best bet is to do the party on my arrival day. Hoping to get the earliest flight out… Get checked in at the hotel, grab some breakfast… maybe swim (or go to another park for a few hours, but probably not) get ready for party and head over to MK. Depending on who my kids MUST mets are ( mine is 7 dwarves, but I think the kids may say Jack and Sally. I was thinking once getting into MK… Getting SOTMK cards for our “regular tix”, maybe play around for a little while and grab a sit down mean at Tony’s for about 4:15ish… And then get in line for Sally and jack. Then continue from there. I’m probably over thinking everything but I get obsessed with planning! My husband hates it lol… You guys would understand tho.
Planning - too much is never enough. 
However, I would not do MNSSHP on arrival day - even with relaxing at the resort after arrival, my crew would be too tired from getting up in the middle of the night to catch the early flight to be able to enjoy the full time at the party.
I agree with @brklinck. Do it the following day. On your arrival day, you should relax, the go to a park for evening hours. Sleep in next day, then get ready and go to MK. Plan in sleeping in the following day. We never made it back to our resort until 1:30 am on Party days!
Looks like I may be adding a RO reservation for the day before… Lol
If I add a RO day before, can I start using my tix days that do or do I have to wait till the next day for the package? Will we get two separate Magic bands?
As of now, you would get two MBs ( who knows if that will change). There have been reports of being able to get tickets up to three days before packages but I have not read that that is guaranteed.
Could I start my tix days even tho the package doesn start till the following day?
If you want to go to a park (I can’t NOT go to a park on arrival day) go for it, just plan things that aren’t “once in a lifetime” or hard to get or hard ticket events like MNSSHP in case something happens, like plane delays or tired kids (or grownups) etc, and you miss them.
And no, never too soon to plan. I start planning the day my deposit is charged (and of course change it up a million times lol).