I swap out GFO once a month, and when things are going south I switch to hi-cap. It is correct to say that rocks will either store or release, depending on PO4 levels. GFO nor carbon lasts more than 2-3 weeks on a relatively low nutrient system I would have to guess, much less on a high PO4 system. When I was bringing my tank back from the dead I used 1/4 of a phosban tube of hi-cap and swapped every 2 weeks.
I've never liked leathers, so I would say "good show" if you lost some. Run carbon, leathers can go toxic when they die!
I've recently taken to doing a 5 gallon water change a day, every day. I'm hopefully leading up to the "big move" so I want the corals ready for an influx of "fresh" saltwater. I'll be running carbon on the tank as of day 1 and GFO within a month.