1. Yes as far as I can see apart from the DXB run as mentioned. Multipletimes through the hubs is fine. Remember that you can only have two stopovers in total in Europe.
2. In my experience BA and QF are pretty similar in FIrst. I'd go for the miles. But can you not fly the BA codeshare on the QF operated flight and get both?
3. CX every time - best First Class in the air.
4. Not sure - sorry - been out of BA for a while.