1. What the GPL says (roughly) is "If you give a copy of Moodle to someone else (with or without modifications you made), then you have to give them all the same rights that you got with your copy." So, you have to give them the code, and the right to change it, and pass it on to their friends. If you just change Moodle for your own purposes, but never give a copy to someone else, then you don't have to do anything in particular.
2. If you wish to share any software or plugins relating to Moodle, then Development:Guidelines for contributed code explains the support that is available from the Moodle community. You can also share things just by putting them on your own web site.
3. Well, you are quite at liberty to to sell Open Source software. For example Red Hat and others do that with Linux, and there are a whole lot of Moodle Partner companies around the world who make money by selling products and services related to Moodle. However, if all the software is Open Source, you cannot prevent people from also being able to get it for free if they are prepared to set it up themselves.
Therefore, you may not want to release all the code as Open Source, but this is a tricky area. (Also, is it fair for you take Moodle for free, and then exploit it without giving back?). What is allowed depends on whether Moodle + other stuff counts as a derivative product of Moodle, or whether it counts as an aggregate of separate products that just happen to work together (with these term as defined in the GPL).
For more information on the GNU licenses, http://www.gnu.org/licenses/gpl-faq.html is a good place to start reading. If the answers are critical to your business, you should probably consult your own lawyer.
Also, you should think very hard about what your business model is. Where exactly are you adding value? To maximise your revenue, which parts of your software and content should you share, and which parts do you have to lock down?
By the way, if you are discussion interesting developments related to the Quiz module in Moodle, the Quiz forum (http://moodle.org/mod/forum/view.php?id=737) might be a better place to say them.