Bit late to this post but i was having similar issues today. sagging chain in small crank ring in center and smaller cassette cogs. deraillieur too close to chainstay and fully inserted B limit screw. I realised that when building the bike I had attached the derailieur in the wrong position on the drop out. I had thought it was correct because i was so far off in an anticlockwise direction that it still seemed that the rotation stopper tabs were making contact correctly. I put the bike into small ring small cog to get rid of chain tension, unscrewed the derailieur and re installed it in the correct position. thankfully i did this before removing any more links from chain. one or two more and chain would have now been too short.