The 2x implementation time was an loose estimate. However, we found that thinking through all of the edge cases, and getting it to work well significantly impacted speed.
In terms of growth in mobile traffic, I have no doubt you are right. However, considering the lifetime of the design - I would be surprised if this number changes significantly before we do another redesign!
We switched from PayPal to GoCardless for one off payments and we haven't found any problems amongst our customer base. We occasionally have to explain the mechanics behind it but generally people get it and prefer it to not having the faff of a PayPal login.