Don't use tabs, launch a new instance for every bot. I think that the client is unable to grab the correct varps in tab mode. If that's what you meant by saying that all accounts are on the same step. That would also explain why you're seeing the concurrent modification exception and the issues that you ran into in the past.
I'm not sure but maybe if you clear dreambot's cache (not talking about jagex cache) by pressing "refresh all" it'll work with tabs. I never used them so I can't tell if there's a solution.
There is no cap, it can complete all quests in a single run, as long as your account meets the requirements.
No, you don't have to uncheck 'GE' if you're running tutorial island. Grand exchange is disabled if you're running tutorial island. Simply start the tutorial script.