Summer + Europe = trouble. Don't even bother looking up loads now, because yes, they WILL fill up between now and then. And on UA, it is a huge no-no to be listed for the same flight you hold confirmed space on, I believe even a different flight on the same route/same day is a problem. Also, if you don't show up for the flight you hold confirmed space on (keeping in mind that your nonrev listing is separate from your confirmed listing, so you'd still no-show as far as the confirmed booking is concerned) the rest of your itinerary (that is, your return flight) will be cancelled.

The one thing I can suggest is that if any city in Europe has seats available on UA, it's LHR. Not really where you're looking to go, but something to consider. Of course, that's not a guarantee, and those numbers you see now will be just as useless so far in advance. But if LHR is full, you're going to have a hell of a time getting out of anywhere in Europe.