Yes, one commission.  It doesn't matter the number of partial fills or days to fill the order as long as it is completed within 60 days.  If you change the order, price or lot size, the order is essentially cancelled and a new order is placed.  Thus, in the latter example you would be charged for a second commission.

It really is a nice feature.  I started using it when I noticed my AON would be passed over even though I saw nobody else on the bid and the trade size that would go thru was identical to my AON order (i.e. my AON order should have been filled in that trade). 

GTC for 60 days is a great option for the end of day trading to avoid paint jobs.