The crashing is most likely due to your OSD and subtitle messages being drawn to the Overlay/VMR surface instead of the image itself. Just figured that one out in Potplayer.
Try having your OSD messages drawn to the image itself in MPC. While that fixed the crashing for me, I still don't get fluid 60fps all the time, yet my cpu is never very high...
Trying to combine your "Processing threads" modification to this to see if it helps (I think it does but I need to watch/test more to be sure).
Using the DXVA-HD scaler gave me the best performance/quality balance when downscaling 4k-->1080p
Using LAV Video Decoder with DXVA2(Copy-Back) ----->huge help on keeping the cpu low
4790k+GTX1070 as well (currently using around 35-37% cpu and 9-12% gpu decoding HEVC 4K w/SVP 60fps)
Let me know if that helps or if you find anything else... there isn't too many of us doing this kind of testing yet...