I empathize with your situation. We lived thru an international trip last year where my MIL became quite ill with surgery eminent as we left. Day by day, we placed the calls, not knowing if that was the day we'd have to figure out a way for my husband to return to the States. In the end, much as you say might happen with your MIL, the surgery didn't occur due to other health issues.

I think you should take it as a given that your wife will want to be with your MIL if/when the surgery occurs. I wouldn't make preemptive changes yet. Your tickets are booked; your cost to change them will be the same whether you do it now or in two weeks, and there is the possibility that the surgery will be rescheduled.

Will your wife honestly truly be comfortable with you staying at your vacation destination should she need to go to her mother's side? And are you of a personality that you can stay and enjoy being on your own? You need to sort thru that, as that will help clarify your options.

I wouldn't buy a one way ticket today due to the uncertainty you face. However, if at the relevant moment, you can still buy a one way ticket for under $150, I'd do that before I drove for 8 hours and paid for the gas for such a trip (the WN option).