Reporting Safari Bugs - Apple vs. WebKit?

Any advice on reporting Safari bugs to apple using Feedback.app vs. reporting them to the WebKit team? It seems like the WebKit people get kind of grumply when a bug is Apple’s fault, but I’m not quite sure how to judge this.

Is there, for example, a browser which uses WebKit but is not Safari to test with?

That depends on the kind of bug. WebGL related bugs should be directly reported to the WebKit bug tracker: https://bugs.webkit.org/

Firefox and Chrome for iOS are WebKit based.