Hi Alexandra,
don't worry too much about your bees! Several weeks' worth of die-off from a wintering hive amounts to a LOT of dead bees, so the sad heap on the hive floor isn't necessarily a sign of disaster. If you tap the side of the hive roughly where the bees are clustered, they will usually give a faint buzz if you listen carefully. Plus, if they're still alive in there, there should be occasional cleanouts (such as the discarded dead bees you noticed) and little showers of wax uncappings as they eat through their winter stores. The hive usually smells nice too: a faint honey scent from the entrance rather than the cold mustiness of an empty hive.
If you feel that they don't have enough food (and we're not yet out of the woods, I'm afraid), then definitely put some fondant in there for them ASAP. Don't worry about the temperature. Just give them a bag or pot of fondant (at about room temperature or slightly warmed) with a slit in the bag or a way into the pot. Don't put it outside or on the landing board. Put it above the cluster where the bees can most easily get at it. Get it into place quickly so they don't lose all their warmth, and put the insulation and roof back on sharpish.
A dark-coloured, shallow water container which will catch the warmth of the sun might be useful to the bees too, although I'm sure they'll find themselves some moisture somehow. Just make sure they can get in and out of the water easily, by floating something on the surface and/or sloping the vessel. They have a strong tendency to drown otherwise, being rather poor swimmers.
I went to give mine some fondant today, and found one colony sleepy and clustered, the other ones as lively as spring - bustling over the combs, tidying out dead bees, and busily exploring the new fondant as soon as I put it in. They vary so much!