1. Anyone can provide help. Anyone. Due to how the combat works, even at a low level, you can stall a monster long enough for a bigger guy to swing in and kill it. And if you weren't around to stall that monster? Entirely possible it could have aggroed onto a guy while he's dealing with a seperate monster, thus initiating a multi aggro situation and turning the entire thing into an enormous clusterfuck. More people is more better, period. Don't worry about dying, although obviously try to prevent it, because if you're alive you're not distracting a monster.
2. Healing and support are more auxilary. If you wanted to you could easily play a combat class with a secondary focus on healing or support, but you should primarily be focusing on combat, due to how the combat works. A monster won't lay off of you just because you're support and fighting more then one monster at once is a risky prospect at best, even for experienced players.
3. There's some truth to it. There are things called reforge tools, and some of them are cash only, and they provide pretty huge usability boosts to reforging in general. With that said, reforges are by and large min-maxing sort've things. I wouldn't call it pay2win, I'd call it "pay2better". You can still easily match a person with said reforges in terms of staying alive and killing things, he'll just have more damage/survivability (the combat revolves around not getting hit, so that isn't as big a deal as you'd think). There aren't any cases of someone plonking a reforge down and SUDDENLY HE'S A TINY GOD AND SOLOS END GAME CONTENT or anything. No matter how you slice it you are working to get to the point where you can clear things.
4. The group is fairly loose, and mostly there to provide background chatter and help if you need it. It's currently in a lull, so yeah, don't be afraid to interact with randoms. Generally pretty well behaved, although it's a F2P game so it can be very hit or miss. Don't worry about "party composition" too much, it's basically all about the combat, support roles just make combat less risky, and healing basically everyone can do to some degree. Not that there's not any benefit to having high rank healing and support skills, of course. It's not like say, WoW where doing a high end dungeon requires this and this and this and this and that.