Hey MissJett! Cool project you have going on there!
Here is what I have come to know on seat recovering. I have a good friend that does many many recovers, and have learned a lot from him.
You are correct on the corn flake seats and the ones you see on eBay, you don't know what they are going to look like when you yank the cover off. One rule of thumb my friend has told me is if the cover is ripped and the foam is missing at all on the top, there might be a problem with the top pan. They seem to get rusty along that bottom edge if moisture gets under the cover and foam. If the cover is in tact they seem to be better.
Also, to fix those seats that have a rusty top pan, I have seen him spot weld in some metal along the bottom edge of the top pan, so there is a nice edge for him to use for the recover. He does this on pans that aren't too far gone, and they usually come out pretty well.
Finally, the best fix would be find an inexpensive Persons made pan and use that to recover. The seats on the 1968 Schwinn muscle bikes had a unique bottom pan, it has a raised platform of sorts where the seat post clamp is attached, and a bolt in the top pan to hold everything together. Those pans are difficult to find for a decent price and in nice shape. But there are many other Persons made banana seats out there that you might be able to snag cheap. The top of the seat will look the same, just the bottom pan would be different.
Good luck with your Mini Twinn!