You can access conventional operation of an Mth locomotive with legacy/tmcc if you have either a Lionel TPC unit or the newer Legacy powermaster.
Yes - and even the older TMCC Powermaster will also operate PS1/2/3 as conventional locos, though the PS-1 sound can get funky using one. It's the wave form and all that; the TPC is better for PS-1 from a sound perspective.
BTW - original Protosound (now nicknamed "PS-1" by us modelers; it was never officially called that) was never a Command system. It was a reverse unit with sounds. PS-2 and -3 are actual Command systems, like TMCC/Legacy, but can only be operated as such by DCS.
I think all that is still correct; it's been a long time since I did Conventional running.