Doesn't blowoff remove nasty fusel alcohols?

Yes, but only a very small percentage of them. Experiments that I've done and described in my article "When Fermentation Rears Its Dirty Head" in Brewing Techniques[33]. show that the difference in higher (fusel) alcohols is not significant between the blowoff and non-blowoff methods of brewing. The main difference is in the loss of bitterness (see "Hops" in chapter 15).

I've read all kinds of different fermentation temperatures for Berliner Weissbiers from 59° F (15° C) to 113° F (45° C). What's the right temperature?

Well, it certainly isn't 59° F (15° C) because the Lactobacillus bacteria will be dormant at that temperature[163]. Although it is the preferred temperature of many lactic acid bacteria, you also don't want to ferment at 113° F (45° C) because your yeast will produce far too many higher alcohols. I recommend you simply ferment at the warm end of normal fermentation, around 70 to 75° F (21 to 24° C) and give beer several months in a glass secondary for the sourness to develop. Alternatively, you could make a gallon of unhopped wort, ferment with just lactic acid bacteria for a day or two at 113 F (45 C) and then boil this wort along with the rest of the extract and the hops - this will boil off most of the DMS that would be produced at that high fermentation temperature[163].

I've read that if the scum on the top of the fermenting wort falls back into the ferment, the resulting beer will have harsh flavors. Is that right?

No. This is a very common misconception. The main effect of allowing the dirty head, as it's called, to fall back into the ferment is that the beer will be slightly more bitter than if you skim or blow it off. This bitterness is no more or less harsh. See "Doesn't blowoff remove nasty fusel alcohols?" above.

What's "too hot" for fermentation?

Every yeast strain is different. Some will make fine beer even at 80° F or more. Remember that the actual beer may be 10° F higher than the room temperature in a high-gravity beer. Try a small batch if you are afraid to commit 5 gallons. Note that if your question is "Can I brew in the summertime?" then the answer depends not only on temperature, but also on how much life there is in your air during the summer. In my house, in the summertime, there is a slightly phenolic wild yeast that lives in the air. In the wintertime, I'm sure it's here too, but probably in much smaller numbers because of the temperature, air circulation and humidity. In any event, in the summertime, the beers I brew have a faint clovey/phenolic aroma and flavor. It's not bad, it it doesn't fit in all styles. I've successfully gotten that wild yeast under control by purchasing a filtered-air aeration system. I don't have to go through all these lengths druring the winter, but if I want to brew in the summertime, I have to take this extra step.