Your room can be changed at any time, I have selected a room at OLCI several times and didn't get that room, for unknown reasons. Sometimes it's because they upgraded me, but equally I sometimes get the room I selected (which was an upgrade from what I booked). Unless you have stayed at a property several times or people have reported details on this forum, there isn't any way to know how a property deals with OLCIs.

I doubt Hilton's IT is advanced enough to show which rooms have been cleaned and ready on the app. As you can choose your room from the day before, when the previous guest might still be there, I don't think housekeeping affects what shows on the app in general (maybe it happens at some specific properties).

You certainly won't get a digital key before the official check-in time since the app doesn't let you specify an arrival time earlier than that. If you are arriving early then OLCI really doesn't do much, it's more useful if you are going to be late as it could reduce your chance of being walked (but if the hotel is overbooked you probably won't be able to choose a room on the app anyway)