I used Ilford Rapid Fixer, which is a hardening fixer, right? So what harm did I do?
My apologies; I noticed "Kodafix" mentioned, but it was in a post by Xia Ke that followed one of yours. The two must have merged in my mind. According to
Ilford's Web site, Ilford Rapid Fixer is a
non-hardening fixer. Thus, your fixing and washing regime was probably OK, although you might want to check the thread I mentioned earlier to see a fuller discussion of the merits and potential problems of the Ilford wash procedure.
Can I re-use the wetting agent?
I agree with others that it's cheap enough to not be worth bothering. B&H sells a 16-oz bottle of Photo Flo 200 for $7. If my computations are correct, that'll be enough for 378 rolls of film at 250ml per roll (what my tank uses for a 35mm roll), for a cost of a bit under $0.02 per roll. At that price, it's not worth trying to economize on it.
Is there any reliable way to control temp?
I think you're getting too worked up over temperature. If your room temperature is just a few degrees off of 20C/68F, use a time adjustment chart, such as the one to which JBrunner has pointed, and tweak your development times. Even the 68-71F range you mention is pretty small. To be sure, using the 68F times at 71F will result in slightly denser negatives than expected, but not so much denser that the negatives would be ruined.
Personally, I use a water bath only when developing color negatives or slides or when the room temperature is very cold or very hot (on hot summer days, say). In the latter case, I don't even bother keeping the developing tank in the bath; I just bring the developer to the desired temperature and then let it drift. If the temperature drifts a couple of degrees during development, it's not that big a deal. Probably somebody who's fussier than I would disagree, but that's how I see it.
To more directly answer your question, though, some people use immersible heaters (such as aquarium heaters) to keep water baths at color temperatures (100F). This obviously wouldn't be good for lowering the temperature, as you want to do. I'm sure you could build a refrigeration rig to do this, but I don't think there's really much need.