Man, these threads have been increasing in number. Roadie here who also rides fixed a bunch.

1. Yes, sometimes. Your 'normal' interval-style training on your roadbike gave you slower average speeds because intervals are not meant to net high average speeds. You burn yourself for a minute then chill for a while. On a fixed gear, the incentive to 'stop and go' is lower since the energy cost of spin-up is greater (no gear-assistance). Over a 30mi ride, if you set a speed limit of 20mph constantly, you will get a near 20mph avg. Whereas if you are sprinting hard at 25-30mph for a minute, then back down to 15mph for a few minutes (rinse/repeat) you're average would be more in line with the 17mph you quoted. It also takes way more effort to do intervals than spinning at a constant 20mph. TL;DR You are comparing apples to oranges.

2. It won't hurt your training. If anything, it will help in some aspects. If you are working on strength and cadence, find some hills with your fixie. You will always be in the 'wrong' gear. Going up you will be in a tall gear - working on strength. Going down you will be in too low of a gear, forcing you to work on high-cadence spinning. Besides the forced pedaling going downhill, you can simulate this on a road bike by just shifting to the 'appropriate' gear. That shouldn't be the only part of your training with a fg, though. Most road racers also incorporate a very low gearing in their fixed training - focusing on high cadence efficiency. That will net more of an improvement on your overall fitness than anything.

3. Yes, they are super fun/no different from a normal century. Make sure your bike is fit properly (drop bars will be HORRIBLE - get road drops with hoods, even if one is a dummy hood) and you eat properly. You'll be fine.