It took me few times read to figure out what you are trying to do, I think I got it.
I think that's a lot of custom cables to make, to get it working like you want it. I'm not sure what signals from RGB are needed for A520, it could be less the 9pins or more, I'm not sure.
I think the best way to do it is to get an External Genlock with RGB PASS-Through, that way you can hook up RGB (using RGB pass-through) to 1084 monitor and composite out to your TV (using video out).
Make sure it's the one with 2 knobs (fade in/out RGB/Composite)
Something like this one:
http://www.bigbookofamigahardware.com/bboah/product.aspx?id=437
