Looks pretty sweet. Regarding your concerns, mufflers are not really "safety" oriented w/ regards to design> except notably for the M3 cars (don't know about other BMW) The riveted tips actually collapsed towards the muffler in the event of a rear end collision.
Your mockup looks pretty nice, but overlay the x-pipe center section more. You want a scavenging effect created here. An example I really like is the Mastermind X-pipe. Google it and take a look at the crossover.
Looks like a fun project that you should def at least make a couple; good luck@!