slow and purposeful means the unit always moves if it was in difficult terrain(rolling two dice and selecting highest as maximum movement)
if the sorcerer dies the squad only gets to roll one dice for their movement instead of 2
Since the codex says 'slow and purposeful (not sorcerer)' this means only the sorcerer is unaffected by this rule but as the sorcerer is part of the unit the unit moves at the speed of the slowest model.
If all the thousand sons die except the sorcerer the sorcerer does not need to make a slow and purposeful roll as the sorcerer is not an automaton so the sorcerer on his own can move 6 inches but can't if there are any surviving TS left in the unit. The sorcerer is an upgrade character(part of a squads) and not an independent character.