Welcome to Battle-Scape. You can join the "Home" clan chat where you can ask any questions you like where the community and staff members (usually Overseers) will help you with any questions or queries, so utilise it well.
Their main purpose is AFK tokens for cosmetic rewards, and to receive passive xp in those skills.
Do the miniquest @ Mage Bank teleport under Wilderness Teleports (it's a safe zone and other players won't be able to attack you unless you pull the lever).
No. And contrary to popular belief, it doesn't boost clue scroll drop rates in OSRS either.
Skilling (fishing, woodcutting, mining). Slayer. Battle-mages and Dagannoths for medium clue's. And puro puro, lucky implings.
Abyssal demons can be killed off task but Abyssal Sire requires a task. Kraken requires task. Cerberus requires hellhound task. There are a few like this, you should list all the monsters you want an answer on as it varies.