I have recently done as you wish to, R671 shifters with Ui2 6770 mechs.
a) you don't ned the combo brake levers, I have the 3 port junction box and the TT shifters only
b) you do need to firmware update. The entire group was brand new to me in Jan 13 (although I do not know the manufacturing date of fr & rr mechs) I plugged it all together and it did not work, (fortunately I had ordered the upgrade box so that I could reprogramme shift buttons etc.), I then plugged in the components to upgrade firmware and only when everything had been updated did it all work together, i.e. fr & rr mechs, and shifters all needed the latest firmware. Battery got a boost at the same time and reprogrammed to fastest shift possible.
However, I would like to know more but the Di2 brake lever and the junction box.
You are using 3 pot junction box and without Di2 brake lever. If I will buy a pair of Di2 brake lever in the future, but now I temporary will use normal brake lever with Di2 TT shifter, which junction box I should buy now?