Some concrete results... The mixed developers work perfectly after about 3 months in the fridge, even when stored in an 70%-exhausted state. My pre-bleach went cloudy (un-refrigerated) over the 3 months, so I mixed up some more of that.

The main issues it that the concentrate bottles seal poorly and one of my developer parts is slowly going magenta therefore I think that careful re-bottling in glass is called for for any concentrate not mixed up immediately when you open a kit.