Update: +1 to everyone thinking 'man, I bitch about google sometimes, but this kind of thing is totally legit and awesome'
Google has generally been good about releasing emulator images prior to releases, though usually it's only a few weeks. Luckily, it takes time for Android versions to achieve large market share, so any bugs can be squashed before it's a big problem.
Not really. It's occasionally convenient to have the source, but it's not at all required and the documentation doesn't assume that you do.
The definitive and fastest answer will always come from reading and understanding the source code, as I don't have to hope that someone had the same problem I'm having and someone else had an answer, or that someone will reply to my question before I give up. Eclair, Froyo and Gingerbread sources were enough for 99% of my needs though, but there was always this shadow of doubt for Honeycomb-specific features...
For the most part, having the latest source readily available will greatly reduce the amount of open browser tabs, and for that I'm thankful :)