Well, this thread having a pop culture bent, I have some pop culture evidence for some matters at hand.
1. According to American Pie 2, there is this rule that guys have where if anyone asks how many girls they have 'romanced', they must multiply the number by three. Thus if they have 'known' three girls then they must up the number to nine. In the case of Hurin he probably slaughtered approx 23 trolls but the rules state he must multiply this by three.
Incidentally the reverse rule is true for girls...
2. According to an experiment undertaken by one Johnny Knoxville, it is impossible to live once set on fire, beyond about eight seconds as you inhale the fumes and so...you die. I suspect that many people watching the films will also have seen Mr Knoxville's experiment and will have known just how wrong Denethor's run was, and this is probably why the cinema was rocking with laughter at that point.
I happen to think PJ threw that scene in as a reference to his old humorous gore fests. There are no lawnmowers in Middle-earth but a flaming Steward was just as funny.
3. Alan Moore was quite happy to sell film rights to his books and then having nothing more to do with them, until a lawsuit over The League of Extraordinary Gentlemen. Now he is not so carefree.