After receiving my printed backplates I am still optimistic 
I cannot say that they don’t fit . They fit perfectly and the diagonal seperation closes both “other halfs” also perfectly.
Thanks to Dirk v.L !
(Indeed I own a jolla phone and already have another “the other half” - TOH) I also took the lower half with spare space for another battery. Unfortunately I still only have one battery and I would rather prefer a bigger one than another FP2 battery. The sony xperias are hopefully slim enough for a more curved out case. I also buyed the bumper frame for FP2. Again using the upper half with the bumper frame fits perfectly and offers me a frame to include bigger breakout boards. However I would need still another flat backplate to close this bumper frame.
I am now expecting breakout bords with FTDI311D + 201 for I2C and some other interfaces . 2 goals should be in reach: increased battery lifetime by more than a double and … the NFC reader would be my 1st choice favourite target for FP2 and Jolla.