I see it as a couple, who have recently met eachother and are falling in love and realise their respective marriages are in trouble because of their meeting, hence the title, "Are We in Trouble Now". It's about the start of a relationship, not the end and I would say it's most likely to be about Mark and Kitty as are "Darling Pretty", "Golden Heart" and "A Night in Summer".
Regarding "Let's See You". Yes , it could be about Ed criticising everything that Mark was doing, perhaps and Mark obviously getting a bit fed up saying "Well, if you can do better, let's see you". Not one of Mark's better songs though imo, but I suppose I couldn't do better either!