I have often contemplated your questions and at one point asked the same questions on the SB forums many years ago. My opinions ...
a) Yes. It makes the mast stiffer but it does not concern me too much. I think the effect is relatively small. If you have the cash just buy a couple of extensions, otherwise don't worry. It really won't cause you to lose the PWA championship.
b) No. All manufacturers basically have a similar minimum length with their extensions into the mast. They wouldn't put holes so high up on the extension otherwise. I am sure there are true engineers working on the design & spec of carbon masts - perhaps not with the brand names like Neil Pryde, North, etc but at least at the mast manufacturers like Italica, Maclean etc. Also the connection between the top & bottom section of mast has a similar length of insertion compared to the minimum length of mast extension into the mast.