So, i have found that currently, there is no S912 amlogic based box that can deinterlace 1080i@50Hz correctly WITH KODI 17 and up (kodi 16 might work). Kodi developers have removed AMCODEC support from kodi.
So to answer my own question:
Can this box correctly deinterlace 1080i@50Hz to a 50Hz deinterlaced video?
NO it cannot. You will end up with a 25Hz judderisch deinterlaced picture which is unwatchable… too bad.