UA cannot ordinarily ask LH to open up award space on a flight. There is a process to do so (using a *A liaison), but for whatever reason very few agents and supervisors are willing to use the liaison at all. You might have marginally better luck convincing an agent to try the liaison route if you have status, or if the cancellation significantly messed up your itinerary. Unfortunately, I don't think an earlier departure and longer same-day layover counts as a "significant" mess.

Are there no other routes from VNO to a *A hub (or any European airport that UA serves) with award availability on your desired travel date? If so, you can ask UA to book you on that and force open I/X space on its own metal for the TATL flight.

If you are not opposed to the early morning SK departure (i.e., the only gripe you have about your newly rebooked itinerary is the 5 hour layover in CPH), you could keep the VNO-CPH flight SK rebooked you on and repeat the above exercise from CPH, e.g. VNO-CPH-FRA-EWR.