Certain classes have skills that enables them to use an ability through a barrier such as the 'Scythe Throw', on the demon class. The current world record holder of Dire Problem did this. From one perspective it looks like an exploit ,however, on the other hand it can be considered a way of optimising a run. Should this be allowed?

It should be fine due to the abilities being made to go through objects, including walls. There should be no problems, unless a run attacks an enemy through the starting barrier.