Knockbacks do in fact stack if you can land them fast enough. Spamming thunderstorm on my shams have seen enemy players punted well beyond the 25 yards - its hillarious to watch too!

If you try to reactivate multiple accounts using the same credit card, you will need to contact Blizz directly - I think it blocks the credit card from being used for activations / char transfers if the same numbers are used 3 times in succession to prevent credit card fraud. I ran into this when I was transferring my chars after my RAF expired.

Cheers,
S.