This brings up some legal issues and a warranty loss. Is it necessary to jailbrake the iOS device first as written in https://sites.google.com/a/webpagetest.org/docs/private-instances/node-js-agent/setup? If yes, which is the safest way that you would recommend?
Yes, there is no way to kill safari or clear the caches without a jailbreak. I can’t speak to the safety of any particular jailbreak or even the availability of a jailbreak for any given version of iOS. If it concerns you then your best bet is to only do device testing on Android.