The 3-Day at the Vail resorts has always been the no-brainer choice for me and my wife. We do it in large part because we don't travel with folks that we can ski with and we are at different levels so we want to ski different things.
At the level you are describing (and frankly at most upper end levels with a good instructor) the instructor is going to gve you plenty of time for play and practice. To be effective, the whole day should not be about introducing new concepts. The instructors at your level should realize that mileage and structured practice are important. So, I wouldn't worry about not getting to enjoy a powder day.
Note that Vail's half days are only in the afternoon. So, the idea of taking a lesson and then practicing on your own may not work as well. I know that it would not work as well for me.
My wife and I have done the 3-day a number of times. We almost always end up with the same instructor for two of the three days, if not all three days. From a long-term perspective, I generally prefer picking a coach and sticking with him or her. That is not what you are doing in this program. You are walking in and getting an instructor you did not select. So, for some people just walking into a random lesson and not having selected the instructor in advance, having the mix of two instructors can be helpful. Sometimes you will click better with one than the other, or one will say or do something that makes what the other said make all that much more sense.
I have never had an instructor actually look at what is on the ticket I hand over to them. I know of others that have brroken the 3 days into 2 days, skipped a day, and then done the third day. These folks have either just done it, or claimed fatigue and been told it wasn't a problem. Last year, at Beaver Creek my wife took a day off from lessons because she wanted to spend the day with my then eight-month old son. She asked the supervisor up front, and he said it wasn't a problem.
The questions that JASP asked are the right questions to me. Were I you, I would absolutely consider what makes sense for you AND your wife. If you can get your wife to do it, as well, that would be great. My wife looks at it, in part, like a social experience. We also still make time to ski together. My wife and I get on the chair at 8:30 and then ski for the hour until we meet the lesson. After the lesson, depending on when in February you come, and whether some of the lifts are spinning until four, you and your wife can ski together again for a couple of runs.
I also recommend doing the lessons toward the beginning of your trip. One, you will get to the know the resort better. Two, you will get the instruction up front so that you will have the benefit of that for the rest of your trip. Three, if your wife is only uncertain about the idea, she won't be dreading the thought of doing it for several days.
From a simple dollars and cents perspective, I also recommend the three-day. The cost of the three full days is still less than three half days, and much less than three separate full days. So, even if you do two full days and beg off early on the third day, it still makes economic sense.
Note that Vail changes their multi-day ticket prices depending on when you are coming in the season. If you are coming at the end of February, I think you can save money by buying the three-day with lift tickets and then buying a second three-day lift ticket, as opposed to buying the three day lesson and the six day lift ticket.