Well, any quest that involved a specific skill give you that experience point.
Plenty, I have listed some for you below.
Some quests give gold, plus you can try fishing, teleport tab making etc. those are good ways of making money.
Some of them you get from quests, some from holiday events. And one you can only do if you are wearing a skillcape.
These worlds are for runescape players to gather together to complete quests together. Some players might want to have some company when doing quests, these worlds (although not proven :p) might increase the chances that you find someone doing the same quest as you. Sone quests in Runescape require partners to finish the quests eg. Shield of Arrav, these worlds might make it more simpler to find a partner.
Some quests where you have to draw something, usually with charcoal. you can buy both of them on Karamja.
Such quests may exist, but if it is farming experience you want, better just go ahead and do some farming. That will be much faster than any quest.
It's a game that is like Runescape but you don't need to pay anything there is no membership and you get skills to 99 in 1-2 days like in Dragonscape. Thieving to 99 took 2 days but in Runescape it took some weeks. Also you can say that it's a kind of copy of Runescape.
No. You can take the shape of some creatures, especially during certain quests, but I don't think dragons are included.
The poison salesman is at seer's village in runescape. He plays a role in some runescape quests like murder mystery. The easiest way to access seer's village is by teleporting to camelot and running west.
According to an online guide, you get "A casket containing 450 coins, an emerald and a gold ring". No quest in RuneScape gives you a large amount of cash, i.e. coins; some quests give you other valuable items; but in general, quests aren't a good way to make money. You should still do them for their other benefits, though, such as getting good teleport options.
Some skills hovering about 30-40 for skills needed for the quest.For combat quests like Demon Slayer, you might like to have 55+ combat level.
Try some Runescape help sites: www.runehq.com, www.zybez.com they should show you step by step how to complete quests.