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. In particular, they should provide at least a partial answer to 3.
The statistics gathering is done by gitdm (LWN article announcing it: gitdm v0.10 available). Thanks to vonbrand for pointing this out.
The repository can currently (January 2015) be obtained with
git clone git://git.lwn.net/gitdm.git
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.