I think the 1tsp/gallon is for wine and mead making. There is very little available nitrogen in a must. There is much more FAN (free available nitrogen) in wort. I've seen the units, and I forget how much nuetrient adds, but FAN in beer is measured in 100s/gallon and in must at like <5/gallon.
With that said, it is reported faster ferments with yeast nutrient added. Also it helps more if you did not make a starter, since the point of nutrient is to help the production of yeast cells. If you make a starter, there is less advantage as there is less need to grow more yeast.
In a 5 gallon batch, I'd probaby go 1 to 2 tsp of nuetrient and not more for typical gravity(<1.050). I've not heard of it harming, but of it not helping at greater levels and thus wasting money.
I've put it into a wine and mead without boiling it and no adverse effects, it doesn't need to be boiled, although boiling shouldn't hurt it, as some of it is 'yeast hulls' or dead yeast.