Seconded. But I'm not so sure we haven't seen him use the Purson family Bloodline Magic. He didn't just "make the prisoners forget everything" at Walter Park. He erased all traces of the Six Fingers from everything, including the memories of people he never met and photographs he never saw. That seems to me like an extreme extension of Detection Warding; he erased their presence from not just the world, but the past. It might also be that he mixed his Bloodline Magic with other magic to do it, like how Clara mixed her Bloodline Magic with illusion magic to create her Toybox, which has properties that neither magic had originally (I.E. sapping other people's magical energy).
Not sure about the disguise, though. Could be he was afraid that someone higher ranking would notice him if he used his DW. Or he may have been afraid of his family seeing him, since we know Detection Warding doesn't work on someone who recognizes the user. He'd also have to show himself to talk to Amduscias, and if they wandered up to him at the wrong moment, as his mother seems good at doing...Well, it could make his life very complicated, and possibly even require him to kill them.