Being only level 66 slayer, I can't really comment on how long it would take etc, but I would recommend going to the Zanaris slayer master. Shilo Village master assignes difficult monsters hence slower exp.
I have lost the gas mask from the plague city quest, and need to enter west ardougne for a treasure trail clue. Anybody have any ideas how to obtain the gas mask?
Well your only option would be high alchemy. Runes are costly, and gaining that amount of levels will be very costly if you choose not to use pest control or high alch.
They simply do not have the time. There are probably hundreds of thousands of reports filed every day. My guess is that certain reports (scamming, macroing etc) get priority over the less harmful ones.