If it wasn't done fermenting, potassium sorbate and campden won't stop it. Usually, if the wine/cider is at .996 or below, it's about done. Sorbate and Campden keeps yeast from reproducing, so that the fermentation won't start again when it's sweetened. It should have been finished, though, by looking at your time table.
I'm wondering if it's not been fermenting, but you have some co2 dissolved in there. If it was gassy when it was bottled, that could explain some of the carbonation but not the popped cork unless you had a temperature change (did it warm up?). Usually wines and ciders are given more time so they degas naturally. Bottling it at such a young age might have been the culprit, unless you degassed it.
Honestly, though, I'm just kind of grasping at straws! My best guess is that it wasn't quite done fermenting, unless your hydrometer readings were stable and under .996.