Surprise, surprise the starfish died.
TRe you have so much going on in your tanks it's hard to know what to advise - you're basically going for the lesser of any number of evils.
FWIW I have a 250 litres, w/ skimmer, 4 fish and I still swap 10% a week, and strangely I don't seem to get half the hassles you guys get. I will admit I'm thinking of one or two more fish.
The urchin will likely be fine in a reef as it will surely have enough algae there, but watch that aquascaping, they bulldozer.