London is certainly worth the time, but Paris is worth an equal amount, if one has the opportunity. You are right to do Venice in September, after the end of Europe's fierce vacation month of Europe. It also sounds like you cannot change the transatlantic flights to "open-jaw", so you don't have to waste time returning to London. (Since you must return to London, it may make sense to plan a full day around a last night there. It is most unlikely that you can fly home the same day you return to London. There are lots of USA flights from Venice and from Milan, for example.)
It would be helpful to be certain that there are family reasons for Kolobrzeg, which would otherwise not be a first-trip destination. Presumably you have checked luggage charges for any "bargain" airlines, and know if they use "tertiary" airports that are harder to transfer to and from.
Don't misunderstand my comments as criticism for your decisions. My point is that it would not be harmful to expand time in the places you have. I personally would not combine Italy with Northern Europe, but you have your reasons. To be specific, Berlin is one of the great cities of the world, and could easily occupy an entire week. You're not even sleeping there, are you? Would you consider one of the major cities of Poland? I'm trying to save wasted time on hotel changes and travel jumps. Please state if you do not wish to visit Germany.