Just my two cents from my experience:
If the airline reroutes you they usually leave the original ticket untouched, just issue replacement coupons.
Also not using the last sector on a xONEx was never a problem friends of mine. Only potential issue could be short checking you luggage to DOH only, but with a long transit you have a good reason to need your luggage in DOH.
Another issue might be that you have two reservations on the same airline around the same time. Not sure how sophisticated the QR system is, but LH for example is able to automatically find these and then cancel one. No experience on QR. You might be safer not entering your frequent flyer information for your new flight or even booking a different airline for your last sector.