I'm sorry about Winston - I wonder what caused their deaths?
As for the anti-fungal agant - I'm not sure... Frogs, as you know, have very sensitive skin - more so than fish - and any chemical residue could (any usually does) have a detrimental affect on the frogs health. I would remove everything from the tank and give it a good clean with hot water and a scrubing brush, rinse with cold and allow it to dry. It is a bummer to pull it apart, but since Winston got ill in it I think it is the best thing to do to reduce the chances of your new frogs getting ill too.
For the mosses - I would wash/spritz them with water and leave them outside to 'air out'.
Starting again could be a good thing as you can incorporate things you didn't think of before - for example: you could use coco fiber instead on garden soil for the substrate?
To D3 or not to D3 - I would D3. From what I understand D3 is usually received by frogs from the livers of insects. Now since we dont really know how our feeder insects were raised - UV exposure...D3 synthesis, etc, I would (and do) include D3 with the calcium supplement just to be sure that the calcium is able to be absorbed. Again I only dust once every one to two weeks to be safe.
Other members will have an opinion on this.
Best of luck