Call scheduling as soon as trip is on your schedule and advise of your intention to deviate, they ll put a "fake" on your pairing front/back/both. Then once you have made your PS5D booking, you need to have the PNR authorized (or you won t be able to check in).
If dh at the beginning, call scheduling to authorize pnr, if at the end call pss.
You can select your seat on united.com as soon as the pnr is created.
As far as waiting till the day before / day of, be careful, if the flight is full or oversold you won t be able to list for it!
You can also do a 2 leg fake dh if needed.
Hotel should stay, transportation to hotel at the beginning of trip is on you however.
On fake dh, you can still expense direct tv or wifi, buy on board food is on you though.