Thank you for your promise :)
I don't perceive Stas as worried at all about Clara. He looked just serious and absorbed. He was just working and/or texting. He quietly went to sleep. Clara arrived at around 2:30 their time. Went straight to bed, he took her in his arms and went on sleeping. A worried man wouldn't have acted like that. He knew where she was and what she was doing. She must simply have been visiting friend or family, like anybody "like us" :).
You may have noticed that they haven't seen Cleo and Axel for quite a long time; probably because C&A are crossing borders for higher ratings that C&S don't want to cross.
Don't worry about them, they're doing fine :) There is a lot of communication between them, they know what they are doing and I trust they won't do things that might endanger their lovely and refreshing relationship. I think they are more and more in love everyday and wise enough to know how far to go in their experiences. We shouldn't worry about them (and after all we aren't their fathers, are we? :)). Does all this reassure you? :)