AFAIK the second stage merlin with the big nozzle would have flow separation and would shake itself apart if fired at low altitude.
IIRC The concept videos have shown separate smaller DRACO landing engines. Also used with Dragon for soft touchdown?
Ah yes, you're right. The concept video shows them using the second stages main engine for a deorbiting burn, then using a heat shield for reentry, then using [dracos/superdracos?] to touch down.
