Go to Kernel coverage at LWN.net and do a search for "Releases", and "Contributor statistics". Also do a search for "Who". There are various articles in that index with titles like (most recently) Who wrote 3.5.
While these articles may not directly answer your question, they are as detailed an answer as you are likely to find on the net, without trying to collect information first hand.
This article does not mention how these statistics were collected, but you could post a comment. Most likely it was done by using
git to query the kernel git repository. If you know the basics of
git, or are willing to learn, then you could use
git to query the kernel repository yourself. I think one could easily use
git to find the answer to 3, for example.
As for 1 and 2, they are not so well defined. In the case of 1, I imagine the answer is almost certainly yes, some of the time. But it is not clear what you are looking for - anecdotal evidence, or some statistics. If statistics, in what form? In the case of 2, by "3rd party groups" it is not clear what you mean, and what kind of help you are referring to. Would people on an IRC channel count as a third party group, for example. Or are you talking about a more formal contractual relationship where money changes hands? Like an outside company retained for temporary consulting? In any case, such information would be hard to get without talking to kernel developers directly, and even then would likely be anecdotal. I suppose forums like the Linux kernel mailing list would be a possibility in that case.