Yes, I believe you can. You can use a cannon at nearly any monster in Runescape. Not all of them, but most. It is best to find a world with not many people in it so you get more kills and so people don't get annoyed with you lol. :p
Almost any monstor below level 100 combat can be blasted by a cannon. I believe some bosses, including the Elemental chaos (276) KBD (276) Kalphite Queen (forget) Bork (226) and a few others can destroy cannons.
However if you want to use Barker Toads in replacement you can do that instead, since they have an HP value.
no
A normal cannon No, but you can use a hand cannon at your own risk that it could explode whilst you are trying to kill him.
on the pvp/bounty worlds
yes
There are no banks in PvP zones but there is a safe of keeping 3 items UNLESS you are skulled.
Scamming, pvp, pvm, dueling, & merching.
Go on a pvp world.
Take a look on the world selection.
While on RuneScape PVP worlds, if you die, you only lose the items you are carrying, not those in your bank, never in RuneScape will a death directly affect your banked items.
Warriors I would think.
I think it is where yo can battle against a player.
No store in Runescape does such.