I think you guys are quite confused with 50i. Yes, you can drop that into a 25p sequence and edit away but the NLE will just deinterlace the material quietly in the background. You can also drop a 50p sequence into a 25p timeline and the NLE will just drop half the frames.
I can guarantee you that the RX100 does shoot in fact 50i.
"Opening it in Quicktime or Photoshop will show you the interlaced lines." If you have interlacing lines then you are dealing with interlacing. Interlacing means that you have 50 frames interlaced into one 25p file. That's how it worked in the "old" days. Those frames are then extracted by either bob-deinterlacing (preserving the 50 frames per second) or just by dropping half the frames - and resolution - away and staying at 25p.