Well, we have to face that riki and bounty hunter are extremely overpowered in this mode; sitting in invisibility and stealing all the kills is a thing that no one can counter. And if you buy sentry wards, and other players will do same thing, they will destroy each others wards. So you have 2 choices here, constantly spend money to sentries while invisible rats buying butterflies and bkbs, or just take that fact that you will do nothing but feed them.
I can suggest either make sentry wards undestroyable or make gem of true sight undroppable. This should solve this problem completely.