But if the blockchain is truly append-only, what stops me from paying the first payment and then disappearing? Even if I pay it all upfront, what if I charge back?
If their attempts to charge your credit card fail then the pay later company can use the usual tactics to get you to pay loans, since they require some of your personal information when setting up the payment: sending threatening messages, adding a bad mark to your credit report, etc.
Sure. People can tell Jesus/God whatever they want to.