Hi Russell,
Ok, that sounds like a redirect bug in the mojoportal registration page, I will look into it.
The issue with PayPal PDT is probably the same as the issue mentioned on this thread, there seems to be a bug at PayPal where the parameters sometimes get duplicated in the url when PayPal redirects to the PDTHandler page. I've added extra handling for this already in our source code repository so it will be fixed in the next release or if you are able to produce your own build from source code you could get it right now. However when I say fix I really mean workaround, the bug is on PayPal not in mojoPortal, and as indicated in the other thread it is not the first time this bug has come up at PayPal, it happened in 2006 and 2007.
Obviously if some of your customers are getting redirected by PDT then the paypal support was incorrect, there is nothing wrong with the button and nothing needs to be changed about the button to solve this, they just need to fix the problem of duplicated pdt params on their end.
The best thing you could do right now is also enable IPN (Instant Payment Notification), this way transactions are posted to your site regardless of whether PDT works. By having both PDT and IPN the notification to your site is redundant so you don't miss any transactions even if PDT fails. It will still send the user a confirmation email and track the order in your site when the IPN notification comes in or when PDT is correct.
Our documentation has instructions for configuring both PDT and IPN
http://www.mojoportal.com/using-paypal-standard.aspx
Hope it helps,
Joe
ps Many thanks for the cups of coffee!