The 737-600 is quite cramped with the new config that I experienced on ARN-BRU. I lucked out and got the exit window with infinite legroom. I thought this was a Swedish domestic version buy you guys tell me it is the new config for intra-Europe.
In FRA it also depends where you land. The UA arrivals is over in C. Some UA flights arrive in the LH B area or the upper A gates. If you arrive in C then I would do the shower there -- if you are going to do it. The UA facilities are nicer and not as long wait as in the LH Senator lounge. But get your food in the LH Sen lounge. If you don't arrive in C then you have a long walk and have to deal with security. If you arrive by the upper A gates then it's def not worth it.
Why didn't you get the FRA-ARN in your award? It's normally dead easy to get C on SK on UA award tickets. LH flights are rationed..........