Update because I DID book STL-LGA/JFK-ATH on the outbound which was the only routing that worked for me on date and miles. 4-hour connection so should be enough time just to Uber door to door.
But I'm really writing because of an interesting development on the return, from BCN not ATH.
BCN-ORD-STL (two pax) was 54k miles + $91.00. I only had 5k left so I bought 49k miles for $1560 thus paid 1651 for two for the O/W.
The exact same flight in lowest class for purchase is 3688/pp or 7376. It's also the least expensive option that day.
I saved $5,725 on a return flight for 2 simply by buying miles instead of purchasing tickets for the same flight? Is this a known hack? I'm certainly glad I looked at the problem this way.