Nightshade gave some very sage advice here!

1) How do you avoid harsh lines of demarcation?
Henna doesn't have as harsh of demarcation lines IMHO, unless you are doing a straight henna treatment and going for something very different from your starting color.
But I'm gonna second Nightshade's advice here.

2) How do you avoid color buildup and consequent two toned hair (bright lighter roots and darker red length).
Try to stick with roots-only applications as much as possible. Henna will build up with repeat applications, and if you're using more concentrated mixes this will be more apparent. I'd say stay away from 100% henna application because you very likely don't need it- you can probably get by with a good henna/cassia blend.

3) IYE/IYO what is the best brand of henna?
I have always liked henna from Hennasooq.
Nightblooming does good mixes if you're looking for a specific color. I use Rusalki.