Yeah I don't think Mackinnon is all that great on the powerplay for the reasons you mentioned.
It will probably never happen but I'd be tempted to put him on the second PP unit. PP2 typically get around 40 seconds at the end of the 2 minutes, and with less time and the PP time expiring there's more urgency to get shots off which would suit Mackinnon's approach better. He'd also be on against secondary PK units or tired first-unit PK'ers, and would be fresh enough to extend the shift beyond the PP to get some more scoring chances.
The setup would basically have the first unit being good at getting properly set up and being extremely mobile with Girard's agility opening up shooting lanes for multiple good shooters (Burakovsky, Rantanen, and Toews) with Landeskog in front to screen:
----------------- Landeskog ----------------
Burakovsky -------------------- Rantanen
---------- Girard ---------- Toews ---------
The second unit would effectively be the "yolo" unit that's more explosive and direct to maximise opportunities in the last 40 seconds of the PP, with Nichuskin and Saad crashing/screening the net when Mackinnon/Makar/Kadri let off quick shots.
------------------- Nichuskin ---------------
Mackinnon ------- Saad ------------ Kadri
--------------------- Makar ------------------