Well if I fly to London on Delta and home on American I can tell you that 90% of the Delta agents are not bothering to try to figure out how to see if my ticket is still valid on AA.
Yes, they MIGHT be able to do that (Depending on the skills of the agents, some of them on some airlines can barely look up their own flights), but they are trying shove several hundred people down that jetway to get the plane out on time. When the plane doesn't go out on time because they were trying to link to some database that's not their primary I can tell you it's not going to be fun for them and they know it. So...
I had to show Delta proof I had a flight home from the Dominican Republic at Christmas. I showed them a receipt on my phone from SW. Agent typed in the date I was coming home and said "have a nice trip"
Now that was my receipt and I was on that plane, but it easily could have been cancelled.
So if you want to be REALLY sure just use a budget airline to book your dummy flight and fly a mainline carrier to Europe. A lot of the budget airlines don't pay for access to the fancy systems. (If you can't find them on Expedia, they aren't paying)