"If Webkit was (pure) GPL, do you think Apple would even consider using it in the first place?"
You've got your history wrong. Apple created Webkit, so they never needed to make that decision. You meant to write "If KHTML was GPL ... " (BTW, the qualification 'pure' is meaningless here; LGPLv2 is not an 'impure' version of GPLv2.)
Steve Jobs has been anti-GPL since the NeXT days, with the Objective C contribution to gcc, so of course that's was a deciding factor for Apple. However, they could have chosen Mozilla as the basis, or purchased the technology from some other company. There's no evidence that without KHTML they would have done a "proprietary browser from scratch."
I do wonder if the LGPL for KHTML is what made WebKit be available in the first place, but that is not relevant for this discussion.
"If you go with a BSD-style license, much more people will use and incorporate that project ... and even if a small percentage of them contributes back, it's still good."
As far as I am aware of, there is no evidence for this assertion. I've been involved with BSD and GPL projects, and the license does not seems to affect the amount of feedback or amount of uptake. In any case, the percentage of good code feedback is usually minuscule; excepting a few large-scale projects. You can see evidence of that elsewhere. Quoting Tim Bird, ex-BusyBox developer "[I'm the] guy who lamented that the busybox lawsuits had never produced a single line of usable code added to the busybox repository."
Your comments here reflect optimism, but that optimism has little base in how software is actually developed.