score:2
The situation you've described is one of the few cases where two one-way tickets will almost certainly be the best options.
With two one-way tickets, if you decide to cancel the outbound flight then you will lose the value of the outbound ticket (you might be able to keep a net small amount as a future credit if the ticket price is higher than the change fee, but that's probably unlikely), but that's all.
With a return ticket, because you're changing the outbound flight, you will need to pay the change fee, PLUS the ticket will be re-priced to the fares on the date you make the change, which will most likely mean that the price of the return leg will be higher than when you originally bought the ticket.
The only real disadvantage of booking two separate tickets in a case like this is if you need to cancel the entire trip. If that happens you'll need to pay the change/cancel fee twice (once per ticket), which will likely mean you get nothing back. If you had booked a single return ticket, and if the ticket price was higher than the change fee, then you would at least get something back as a credit - even if it wasn't much!