1) Is it possible to effectively seal lock someone and slowly drain their HP(Rappelz Gold) with dots and Explosion of Petrification?

Using your DoTs in duels is only part of the total damage in Rappelz. You will be able to drain a lot of their HP with just DoTs (depending on your M.atk since Huna DoTs are a percentage of their M.atk). You will also need to mix in the CC skills to do a huge proportion of damage.

2) From a R5 or higher standpoint, around what general enchant should 2H Axe and 2H Staff be to easily stand your ground against others? We realize this is more of an opinionated question more than anything, but we are just trying to figure out how much we'll be investing into this class.

Don't take the axe higher than +10, its just not worth it. This is a exp from freshly r6, his 2h axe is currently +6, and we are not sure he can going to take it much higher.So your staff, on the other hand, should be +'d as high as you can afford. We also carry around a clean 1h axe and a +6 or better shield.

3) Druid cards are expensive as hell, which PVP-focused skills should we be investing cards into? we know not all cards are useful, seeing as how some only reduce MP cost but MP seems to be a big problem in long duels.

Yak advent and explosion of petrification are very good choices for investment, the cards are relatively cheap, they're skills you use a lot, having them less MP expensive is nice (and the extra damage on petrify), but the real reason you want to + these is the hidden accuracy adjustment. We've found that with both these skills at +3 they almost never miss, only on full wis build bishops and higher level(rappelz rupees) chars.