If I roll a pvp guy for the combat quests...

...Beat em, get the reward, store the coins, can I delete the slot, re-roll it and do the pvp quest again fore more coins indefinitely?

Yes, you can. Until the quest is changed. But then you just switch to doing the new one, and you can still create, delete, ad infinitum.

That's what i did yesterday for hero battles to farm balth faction (2 wins is ridiculously easy)

enjoy it until they make it per account to stop it