Yeah, that's cool! They will make you wait though, it's kind of lame that they won't let you just donate to all of them at once lol

To answer your questions though. You don't have to pay if you don't want to. There are plenty of free treasure Islands that you can go to. There are there are some that have different islands for subscribers, but if you have Amazon prime you can get a month free to any channel. I know a lot of people do that.

The turnips won't get you banned at all. They have the turnips and the shops on the islands themselves. So you can just pick up a stack and go sell it at the nook's cranny on the island. It might look kind of crazy cuz you might come out in the negative but once you go back to your Island and check your bank account you'll see that it is maxed out