Your filter sounds like a good one. Even though your readings are staying stable I would leave the filter alone until the water flow slows down through the outlet and keep the dirty water in the filter.
You may want to get a bucket just for fish use, traces of cleaning agents can be left in sinks...although im sure the chances of transferring anything are minimal...best to be on the safe side though.
As for your feeding as long as there is no left over food all should be well. Fish remind me of my dogs the more they eat...the more that comes out the other end (keep up those water changes)
Im sorry to hear that. In my experience with dropsy the poor wee fish are too far gone once their scales start to protude.
Regular weekly water changes are fine as long as the new water has been prepared and the water change isn't too large.
Your filter is the perfect breeding ground for good bacteria and every time you clean it some or most of it will die back usually leading to a small spike in water parameters untill it grows back. I only ever clean my filters when they start to clog and when I do I use a bucket of tank water to rinse the sponge. Never use tap water as chlorine and chloramine will kill filter bacteria.
If your feeding twice a day and there is no left over food on the substrate with good water test readings then carry on (as long as each feed is a small one).
Unfortunately it definately sounds like a case of dropsy.
This can be brought on by a bacterial or viral infection but also by a weakened immunity.
Usually if bacterial or viral more than one fish will be affected in quick succession.
Weakened immunity can be brought about through poor nutrition, poor water quality, genetic weakness etc.
Your water quality seems good, however, I tend to replace my test kits every six months after they have been opened. It would be worthwhile checking your ph as well as any problems here can be just as detrimental.
A weekly 10-15% water change should be adequate as long as you don't overfeed if your tank is overstocked. Make sure you fully dechlorinate and check the water temperature is roughly the same as your tank before you add it in.
If your hospital tank hasn't cycled I would be inclined not to move your fish as this may cause added stress. I would continue to moniter water quality and treat with Ip No9 Anti Bacterial.
Make sure to feed a prepared food designed for fancy goldfish. You could also try feeding a block of defrosted bloodworm on alternate days to the prepared food.
I think the idea of adding the extra female is the safest bet. If your going to move anyone, move the male...or better still use a divider. That way they still see each other and their tank-world doesn't get flipped upside down.
She will eat enough to keep herself going until she releases the fry & she'll make up for it after that.
Any food she eats will also help feed the fry as the smaller particles are vented through her gills.