Reporting Safari Bugs - Apple vs. WebKit?

Any advice on reporting Safari bugs to apple using 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:

Firefox and Chrome for iOS are WebKit based.