The BMWPE comes in two parts. An axle-back which is a muffler and pipe that runs under the rear axle and connects to the midpipe via a flange. The second part is a resonator delete, it connects at the flange to your axle-back but then requires cutting of the midpipe to remove a resonator that is in it. This pipe is also referred to as the Y-Pipe due to its shape and is totally a legitimate part of the BMWPE system.
If the dealer has not retained the original midpipe resonator then you can by all means fit the stock axle-back portion of the exhaust as it is 100% the same as the BMWPE axle-back. You will of course still have no midpipe resonator so the car will be a little louder than a totally stock car.
Personally if the BMWPE isn't your wife's bag I would suggest selling your exhaust to a local member and arrange cutting of their midpipe so that you may have their resonator and they may have your Y-Pipe. You never know though, you may like the tone of having no midpipe resonator and stock axle-back, deciding just to sell the BMWPE axle-back which is worth quite a few $hundred.
BMW 135i N54 | Black Sapphire | M-Sport | BMS 10mm Spacers (Rear) | Eibach 5mm Spacers (Front) | Gunmetal 261M 18" Wheels | Vredestein Ultrac Sessanta 225/255 | LCI Tails | COBB AP Stage 1+ | Matt Black Grilles | BMW PE + Resonator Delete | N55 Midpipe | Rieger Tuning Matt Black Diffuser | ETS FMIC