Okay, partial success here. First, I did not use paralelled outputs with floating controls. I used two separate gates, with remaining two gates floating. Now I soldered some jumpers, so there are 2 gates paralell per channel. Picture is much much better than before, but still dark and grainy.
Do you think guys I might just stack more chips? Say, one or two atop existing one? This works with motor drives or transistors arrays . With every additional chip, RON should decrease by paralelling...
I checked local suppliers and I couldn't get dedicated video switch IC, neither suitable opamp for buffer. I want this working ASAP, so no chinese chips from ebay for me now. As the last resort I could just buy some small relays, if stacking idea won't work