On the E-jet the MCDU is much more than an interface with the FMS. The problem your facing is one of procedures straight from Brazil. If you separate the other MCDU tasks from the pure FMS setup then you will find 6R works fine.
It's the work arounds introduced to compensate for software bugs that are getting in your way. As the bugs get fixed, and believe me plenty have been. The work arounds will slowly disappear. With load 21.4 there is no need to reset fuel used and FMS AUTO can always be done with instrument setup so in fact you could just 6R now and be fine. But until the SOP is changed the training provider is bound to go by the book. Your airline is free to approve it's own SOP and could clean up a lot of the Brazilianisms but it takes a bit of background knowledge to know why things are like they are and the E-jets can be a bit complex at times so be careful.