A few facts about the Cinderglacier (CG) proc:
(a) Death and Decay (DnD) does not consume CG charges but benefits from CG.
(b) CG increases the damage of both Frost Fever (FF) and Blood Plague (BP) if it is active while they are applied. CG stacks are only consumed if the triggering spell itself benefits from CG. That means:
Icy Touch consumes 1 stack of CG, although both Icy Touch and the triggered FF benefit from CG.
Plague Strike does not consume any stack of CG, but the triggered BP benefits from it.
Pestilence does not consume any stack of CG, but all diseases created by it benefit from it (the original diseases remain unaffected, however).
Chains of Ice does not consume any stack of CG, but the triggered FF benefits from it.
(c) CG's bonus is updated on-the-fly for DnD, but not for diseases. That means if you get a Cinderglacier proc after you've cast DnD and applied diseases, your DnD's damage will increase, but your diseases's damage will not. (The same is true for the case that CG fades: DnD's damage decreases, FF/BP's damage remains unaffected.)
(d) CG increases any Frost and Shadow damage from item procs without consuming charges. This applies to both direct-damage and damage-over-time procs.
