1. The 340i has a turbo as well and the whole exhaust system has a bigger diameter / is designed for more airflow. Actually the silencer itself has not much impact on the backflow since your DPF is the element with the most drag in the system
2. In this topic you will find links for adapter pieces, 10-20€ on ebay. What most of the guys did was welding the adapter tube onto the silencer and then connecting exhaust pipe and adapter with an exhaust repair clamp (links also given in the topic). In my opinion an exhaust clamp is always a "weakspot" in the exhaust system for corrosion, rattling noises etc so welding is the "better" option for the connection of pipe + adapter
3. The mounts are different. Either you order the new ones or you have to bend your old ones properly
4. Those vibration absorbers are specificaly designed for a certain vibration / frequency, so the one from the 320d silencer will have no impact or even worse impact on the 340i silencer. From BMWs side there is no absorber used on the 340i exhaust system. Indeed the diesel engine obviously has more vibration so be aware of that (regarding spacing between exhaust tips and your rear diffusor). For a suitable vibration absorber you would need some rocket science engineer who can calculate the vibrations / wavelengths and weights of everything 
5. Maybe a tiny bit, but dont expect much from it. This mod is just for optical purposes