Hmm that is hard but I'll try to breakdown what I think:
Yatchman- this was never on my radar until I saw someone ask if this is part of the DDP. The steak in the menu description sounds like the one they serve at V&A. If and only if this is included in the DDP, I think you should go for this. I would actually rearrange my plans if this is included too
Flying Fish- I really want to try this so I included it in my plan. The reviews r very good from what I can see despite peopel's dislike of the menu change. but does the menu really appeal to you and your son? As you said, the menu of all signatures are very similar to one another now. Is it worth changing la hacienda for that?
Cape May- I've never been so I can't comment. I think that if your son likes it, you could swap it. I will say even if it is close by, it is much more convenient to be in the park already.
As for dressing up, I think it's easier to dress up properly in December since it should be cooler. Dark pair of jeans and a clean shirt would be fine. On the day we're planning to do flying fish, I'm planning to be at Epcot that afternoon to go around WS and head to flying fish after. We would not go back to CR to change. So I would dress appropriately and make sure my son has a change of clothes just in case
Another point I wanted to say was that your son was looking forward to la hacienda. Is he going to be disappointed if you change that? Would you be able to squeeze it in at an off hour just to get him the cheese he wants?
On another note, I'm currently looking for flights now and my choices aren't ideal. It's either 11:55am, meaning I have to leave by 8am or 9:55pm and arrive back home past midnight (and head to work the next day). My DH is leaning towards 9:55pm. This would give us another dinner and I'm really considering this. But it seems stressful too... And I will quote you- decisions decisions...