I am not arguing that a Knipex pliers are stronger than a similarly sized pipe wrench but I do believe the design and actuation of the jaws and teeth create more grip than a pipe wrench... again from my experience... no data, sorry. In fact, looking at Mc/I stress, a cheater pipe may fail the Knipex handle before a decent quality pipe wrench handle, but I don't think strength of the tool is the issue.
I will vote again for Knipex and heat. A little cheater won't hurt. Looking at the OP posted pics, I would say grip is the primary issue... let's get a better grip and add some heat.