Always worth remembering whether a forward is starting or finishing is usually an argument over 5 to 10 minutes of playing time
It's generally a bit more than that. Some of the front rowers normally get replaced first and at 45 minutes that's a 10 minute gap in playing time.
If you replace a lock at 50 minutes that's 20 minutes (I would predict the plan would be to take Arnold off and bring Rodda on).
If McMahon is on the bench he'd most likely replace Timani at the 50-60 minute mark.
Personally I'd be starting McMahon at 6 and then moving him to 8 when Timani is cooked.
If the lineout is struggling Hanigan can come on a bit earlier for Timani.