Doesn't necessarily mean that it's sold out. Could just be they are holding it for brewers and will open it up at a determined "Registration for Homebrew Con" date/time.
Safale 05 usually has an apparent attenuation of about 80%, and 04 just a bit less. How are you figuring your FG? With software or are you doing the math?
Well, to answer the question, we're going to need more information. What is the grain bill, typically? What yeast? What mash temps and times? Fermentation temps?
A dehumidifier will not handle the steam from a 5 or 10 gallon rolling boil. And, a condenser will not hinder DMS or "other nasties' from escaping. The steam is simply converted to water by the condenser, and the water (hence the steam and other nasties) goes into a bucket or a drain.
Yes, as others have said, go with a steam condenser. You will not regret it. Not only will it handle all the steam, but it will also allow you to boil using less energy, causing less stress on your wort. Win, win win!