.
2) You can use your dining credits anyway you want from check in until midnight of your check out day. You can use them all in one day if you wish to do that. You can use them for any QS meal breakfast, lunch, or dinner.
I'm not sure #2 is entirely correct either. I'm not an expert on the US QS dining plan, but my understanding was that, unlike WDW, Universal QS Dining plan credits don't expire. I think the meal credits on their Table service plan expire, but not the QS.
When we had it, we had two types of drink credits: one was for the mug itself (including use of the mug on the day we activated them), and a second type of credit that was for activating refills on subsequent days. On the first day, we only activated half our mgs. we were able to activate the rest of the mugs another day.
When activating a refill credit, you will likely want to do it early in the day to get the most use of the credit. We only ever used the QS dining plan once, so I'm no expert, but I know we activated a refill mid-day. I couldn't say what time it expired (park close? midnight?) but our refill activation did NOT extend into the next AM (so not 24 hours).
A nice thing about Universal is that they have multiple drink stations all over the parks and include ice water. the downside is that you have to carry the mug around, which generally =having to put it in a locker for most rides. Most of the time, I don't want to deal with it all day. I very much would not want the hassle of having to deal with 3+ mugs all day. If the cup was collapsible, I'd do the plan more often.