PayPal will be fixed in 6.5. You can always run 5.x if you want to use it now. It'll work OK.

As for WhatsApp: short answer is "not happening". It was re-written to rely on Shared Workers, which is essentially a hidden browser that runs the "application" and communicates with the visible tab. I mostly had this working in 5.x, a HUGE amount of work, but there were two problems:

speed - running an invisible tab doing background processing simply made Wayfarer crawl (JIT would help, but not enough).
memory/perf - allowing websites to run Shared Workers would impact Wayfarer's perf in general

In other words: use your phone.