If by flicker you mean a sort of vertical shimmering, that may be due to bob-deinterlaced 480i and is normal for line-multipliers like the RT2X devices (and OSSC). I don’t believe any of the mCable products would be able to deinterlace bob-deinterlaced 480p, so, while those could take care of 480p->1080p upscaling, they wouldn’t do anything for that shimmer/flicker side-effect. You may want to try enabling either the smoothing or scanline filter; I’ve heard those can help mask this problem.
Scalers (as opposed to line-multipliers) tend to include some form of deinterlacing, which is likely why 480i would look better through your generic scaler; however, that comes at the cost of about a frame of display lag.
The darker image from the RT2X may be due to a number of reasons. If you have the scanline filter enabled, that would cause half of the resulting image to be black, which would warrant some brightness compensation; or it may be that your generic scaler has color/brightness overdriven.