I've done some CODM programming but I certainly don't claim to know everything about it. Here's my take on the questions you raise:
1. Is there a list of what can and can't be done in CODM versus a full boat ODF?
I don't think there is a comprehensive list of what *can't* be done in CODM, just a discussion in the manual of what *can* be done. In general, CODM treats all the samples of a rank as a batch. That is, all the samples have common voicing parameters, although the parameters may be varied linearly across the compass. The full ODF description allows each pipe to be individually specified if desired, and special parameters such as cross fade timing can be adjusted.
2. Is the default GUI fully functional regarding mouse clikability and midi assignmnets?
I'm not sure the CODM GUI is "fully functional", but I haven't personally encountered any limitations beyond what can be done with a full ODF.
3. I plan to learn CODM in order to customize an existing users CODM creation.
Are these files third party editable without permission from the CODM file creator?
Third party ODF's which were created through CODM are fully editable, but it would be certainly be courteous to ask the author's permission. In the case of my extended Velesovo ODF's, I have no objection to anyone using them as they wish. Since I do not intend to become a commercial sample set producer, I have no "territory" to protect! I just have fun with Hauptwerk and am happy to share things with other users.
4. If a single sample is also used elsware in another division, are all the voicing parameters independantly voicable?
I'm not sure what you mean a "single sample". If you load a sample rank for one division and then "borrow" the rank for another division, then the voicing parameters will be common to both divisions. If you load a set of samples more than once, into separate ranks, then the parameters can be individually set.
5. Can a single sample be extended an octave for duplicate use elaswhere if the original pitch wasn't what is desired?
The answer to what I think you are asking is "yes". For example, in the extended Velesovo I offset a duplicate set of samples from the Great 16' Fagott and formed an 8' Oboe stop on the Choir division.
CODM appears to have a bit of a learning curve, and answers to the above will assist in the decision to immerse myself in CODM Geekdom.
Please don't call me a Geek!