yes im pretty sure that exorcism spreads on collision, however there is a gloabl timer that starts at every hit. (E.g. you hit the initial target with a total duration of 20 seconds, and after 10 seconds there is contact to a second enemy. The second enemy will now have 10 seconds left of exorcism dmg)
I am usually playing this build almost entirely lazy, that is:
i hit and retreat against bosses, as they will take dmg anyway (works like a charm against astro, qt etc). Farming k3k is the really easy (to bad i have heard that the reward is bad) you just use vessel of judgement + bear stance and everything dies just by walking past them.
The total charm nerf might have hurt this build, i dont know if you can still have a duration of 30 seconds and 10k dmg ( Which after this interval equals to
30*5*10k = 1.500.000 dmg) i remember that in earlier patches i could 1 hit kill d baal while just relaxing and watching him die.