Okay, lets set a few things straight:
while i'm typing here now, I'm booking with aeroplan an open jaw return from the usa to europe, with 5 flights in europe (including a 5 day stop, which is not the open jaw part).
Their taxes are usually very low, and if by mistake they are higher, you just call again a few hours later, and all will be alright.
The best is to mix airlines, not just use LH (or LX), then you have almost a guaranteed failure in their tax calculations.
The SQ FRA-JFK you will not be able to book in First.
The overnight stay in IST you can get a free hotel (shuttle to hotel, and limo back for free) [ask for the
www.grandyavuzhotel.com].
In Istanbul stick to the Sultanahmet area, which is the most interesting.
If you are dressed with the flow it'll be alright. Remember, Turks aren't Arabs, and don't dare call them this.
Worst come to worse, you buy a nanach yarmulke, but without the writing, and youll blend right in with your beard. Just wear slippers to make it complete.
Btw, they just got the taxes loaded. incl their CAD30 its CAD174, which is excellent for a 6 legged trip.
In Venice Id stay in the Ghetto Vechio area. The point is being in local hotels, to feel the vibes, not in a too posh western standard hotels.
Stay a little longer then shabbos, i.e. have say the whole friday and sunday to tour. (there is a eruv there).
Paris depends on your wife and your own love for art, architecture and the likes.
(not wanting to judge, an average american would be fedup after two days, as "everything" looks the same to him.)