Going Below Optimum Yeast Temperatures (For Ale in Particular)

Fermentation temperature - Keep it constant. Encourage the properties you want to accentuate with the yeast to complement the style. Monitor the results, that way if you think the recipie is worth trying again, make the relevant adjustments, like you would with the recipie. Small temp variations have a great impact on results. Phenolic yeasts should be free to rise and fall on their own account preferably within a predictable/constant ambient temp. Too low, clear beer, what about character then?!

You're measuring the temperature of the swamp cooler water and not the beer, and the beer will take a long time to reach the cooler water temperature, which will also rise in temp during the same period. This is because heat transfer between the beer and the cooling water is slow, since the surface area is small compared to the volume, plus both liquids are not in motion.

So, what I expect will happen is that the beer temp will drop in a few hours, but no where near the drop from 75F to 60F. Probably somewhere in the middle - 66-68F at a guess. So I think the yeast will be fine.

But what would happen if you really did drop the temp to 60F? The yeast would drop out of the beer and stop fermenting, leaving a high FG/low attenuated beer.

After the first 3-5 days of fermentation (depending upon activity level) it's a good idea to slowly raise the temp of the beer to 72-74F so that the yeast are encouraged to continue fermenting and not drop out. This will give you a better attenuated beer that often requires less conditioning time. The increased temperature doesn't give the off-flavours you mentioned, since the bulk of fermentation is now complete - the yeast are much less active and produce far fewer flavor compounds.

EDIT: As you see, Denny and I have different perspectives, and unless you have a probe immersed in the beer, you don't really know what the fermentation temperature is. While one yeast strain may tolerate a drop to 60F or lower, another may stall at that temperature. Either way, it's not usually a crisis, just rouse the yeast and raise the temp and fermentation will typically pick up again. But ideally you should be looking for a more even temperature - avoid temperature swings. You can try adding just a single bottle more frequently, rather than dumping 5 in at once.

The yeast will not necessarily drop out at 60F. I've used 1214 at that temp many times and I find it preferable to fermenting it at higher temps. In fact, I prefer fermenting pretty much any ale (and many lager) yeasts at or below the lower end of the temp range. You get cleaner beer with fewer esters and phenolics. I often ferment WY1007 and 1728 in the low to mid 50s, despite the manufacturer's recommendation of almost 10 degrees higher. IMO, all the temp ranges listed by yeast companies are higher than what I prefer. Wyeast sells one strain of yeast that I gave them (WY1450) and the temp range they have for that is at least 8F higher than what I actually use! The exact behavior will depend on which yeast strain you use, but in general there is nothing wrong with fermenting somewhat below a yeast's listed temp range.