I was just reading up on bee sings because another person on here had a duck that may have been stung and I found that ducks can be Allergic to bee stings. She May have been allergic, gotten stung by a bee and died.
That is so sad. I'm sorry that happened.
I would find out if you could with An autopsy to see the cause of death like you said " for the other ducks"
You could use paver stones or make a ramp our of wood with shelf liner attached to it so they have grip.
Again so sorry