I am guessing that was a joke! The Divemaster course will make you a FAR better diver. 100 dives in 12 weeks as a DMT makes a massive difference to your dive abilities regardless of how good a diver you are when you start.
Yes you can just pay the fees and not the insurance. If you go a year or 2 without paying the fees even, and then renew, it is not difficult. Longer than 2 years and some retraining may be required.
This is all dependent on where you do your training and if you do an internship or not. The actual course won't teach you a whole heck of a lot but you'll gain some experience on doing skills demonstration quality and assisting with classes.
When I took my DM, I didn't learn much if anything in classroom and my only diving was the skills, assisting with the classes and doing my mapping project. Was I a better diver than when I started the class? Not really, but I sure could demonstrate how to do skills while kneeling!