ACARS MU switches automatically between the VHF and SATCOM. Indeed the pilot can change VHF3 from DATA (default settings) to VOICE. But ACARS MU would then switch to SATCOM which there is no reference to a switch in cockpit to turn that off. At least I haven't found one
B777 ACARS can communicate on text basis with ATC. Pilots can transmitt/confirm/receive messages trough ACARS ATC. This in case that there is a voice failure.
ACARS of the B777 is highly sophisticated. It is much more than a dummy text transmission tool.