Don’t forget the old 390 video drivers are also available in Manjaro so in the worst scenario you could use that probably if your 850M card is supported by this driver (//EDIT: it seems it is so no problem Linux x64 (AMD64/EM64T) Display Driver | 390.144 | Linux 64-bit | NVIDIA), and there will probably be workaround when the time comes to keep the 470 drivers (like there is for the 340 older video driver currently).
There is also the AUR and I’m pretty sure people will create what is needed to have something to fallback on.
I’m not sure about that, it should work I think only if you hold back the kernel too. But then you are holding back packages and Manjaro would be in ‘partial upgrade’ state, and this is not recommended (not supported).
I would say wait and see, when the time will come you will have solutions to keep a viable Nvidia setup. There is always a solution.
//EDIT: so in short your GT 9500 is supported by the 340 driver that you can manually install following the tutorial in the link above, and the 850M is supported by the 390 driver directly from Manjaro. You will still have support when the Nvidia rolling driver in Manjaro (currently the 470 one) will go to the next version.