I am positive about the midnight cutoff for using the dining credits. We just did it in August. We saved a table service credit and used it for dinner on our check out day.
Wifey...I don't know too much about the QS dining plan. On the regular plan, if you are there for 14 nights(nights being the operative word), you would get 14 TS, 14 CS and 14 Snack Credits for each person. How you use those credits is up to you. You said that you arrive in the afternoon. Chances are you will not use either a Table service or a Counter service credit for that day because you arrive later in the day. You can use that credit whenever you want. If you want to have two table service meals in one day, you can do that. They don't require that you use one credit from each category per day. If you want a day of just snacks, that's okay. The restriction is that you only have 14 credits in each category for LENGTH OF STAY up to midnight on the day you check out.
Hope that makes sense. Just remember...it's up to YOU how you use your credits over the 14 days.