Use the same potting mix it was in last season. It should be fine. All it needs is for you to give it water. The only signs of death would be excessive wrinkling, or soft or mushy feel to the corm.

the old mix should be OK. If you want to give it some fresh soil, just make sure that there is extra perlite or coarse sand mixed in to greatly improve drainage. These plants need excellent drainage.

A dead bulb will be mushy, or perhaps very lightweight. Yours sounds fine. You'll see little green sprouts coming up from the bulb. It's a little early for growing callas, don't give it too much water or you may rot the bulb. If fact, I would later it once and then not again until the first leaf breaks the surface of the pot.
Good luck! Dark Eyes is a pretty plant.