I'd go with an earlier time, just to keep your options open.
It really would depend on how long you wanted to rest and how early you wanted to be at the party. With lunch around 1:30 you should be finished around 2:30. You could be back to POR between 3:00 and 3:30, depending on how long you wait for a bus and where your room is/which stop. You can get into
MNSSHP at 4:00 with your party ticket, but obviously you couldn't go that early if you wanted to rest.
Decide what time you want to get to MNSSHP. Count back an hour from that for transportation time and getting into the park. Figure out how much time you want to allow for a rest and getting ready to go to the party (I'd say 1 1/2-2 hours if you aren't swimming), and count back that amount of time. Then count back about 45 minutes for travel time from DHS to POR. That will give you the time you want to leave DHS. Go back at least an hour from that time and you'll know the latest time to make an ADR.
If you have an earlier ADR than you need it will give you more flexibility in choosing what time to leave the park, instead of cutting it close and feeling rushed.