First, if you have miles, you may want to consider booking this as an award, if you can find reasonable availability. Be sure to look at partner awards since they tend to have better availability at more reasonable prices than AA awards. Awards are one way and fully refundable which seem like useful features for your situation.

Having said that, if you book a round trip for cash, and assuming that your ticket can be changed for no fee (don't book basic economy, obviously) but is nonrefundable, if you cancel the return your trip will be re-fared as a one way and you will get a credit for whatever's left over, if anything. As you point out, one ways are much more expensive than half a round trip so you won't get much back. It will be as if you booked a one way originally.

If you change your return after you start travel, it should be refared using historical prices. I'm not completely clear on ticket validity (one year from start of travel vs one year from original booking date -- I think it's one of those but I'm not sure which one)

Seriously, doing this using awards is by far the easiest way if there's anything available for a reasonable amount of miles. I would spend a long time trying to make this work with awards before considering buying a ticket. If you can't find anything with AA or a partner, look at other alliances and see where you can transfer credit card points from.