On a cap, whether floating or hard, we don't know what the effect would be. I can tell you that if the government moved close to doing it, I believe the banks and the other credit card issuers would say they're going to cut off half the customers who have credit cards currently because they wouldn't be making enough money under a restricted, regulated interest rate. But they could easily be making it up, because right now they hold all the numbers. They know what their profit levels are and what all their costs and revenue streams are.
That's why you need to do the audit first. Anyway, it should be an annual audit looking at increases they may propose in the future and also looking backwards at least 10 years to find out the extent of gouging in the past. Then you could set a cap that would be workable.