Interesting theory about the iPad, or browser, being the issue. Granted I've not had the problem when using any iOS devices.
You raise some compelling points. Granted, I'm not sure how Zen Cart could be changed to accommodate issues that appear to be specific to PayPal's security.

Has the same customer tried the exact same purchase from a desktop PC without the error?

In ZC 154 the 10417 error is only "handled" by Express Checkout, and reports an insufficient funds message.
In ZC 155 the 10417 error is also caught by Payments Pro, and reports "having a problem authorizing the card; please try another".

Another thing to consider: in v155 we implemented an iframe/ajax approach from PayPal called in-context-checkout. This might work better for those customers; but as I can't duplicate the problem I can't be certain; I just know it uses some slightly different factors so could potentially work better on mobile devices (since that's what in-context was designed for).