I'm going to use my miles for someone's ticket
I'm Diamond

The First flight XXX-DTW is fixed on 1/4/2015
The return flight date is unknown

I'm thinking of booking the return 3 months out
ie XXX-DTW on 1/4/2015 and DTW to XXX on 3/30/2015

Once I know the return date can I change the 3/30/2015 flight to this date even after departure of outbound and if so Can I make multiple date changes? (assuming I obey the 72 hour rule)

The alternative would be to wait until 1/1 and get a 1 way - Assuming these are open then