1. Only the hotel staff will be able to tell you if you "pay" for CL access only 1 day out of a longer stay. I've never asked a hotel for that type of access. I'd be interested in what they say.

2. I had a similar offer for the Grand Caymon RC a few years ago. Their offer of $100 was 'per room'. So both my wife and I would have access. I would suspect that your offer is the same.

3. I do believe that RC lounges tend to have food and beverages out pretty much the whole day. And the food offerings tend to be a bit more substantial than what a normal Marriott CL would provide and I do believe that it does include alcohol, but not positive as I'm not really a drinker, so I've never really paid that much attention to it. Note that while the food may be better than what you would find at a Marriott, I doubt it would be steak and lobster either. Its probably more like finger foods and other snacks.

So is it worth it? That depends. If want to try to have all your meals there for the day (breakfast/lunch/dinner) and some drinks. I guess you could figure up how much you think you would spend elsewhere if you didn't get the lounge. Basically, you get meals and some drinks for the $100. Most anything else, you can already get at the hotel. So is that food/drink worth it to you?