Glad that Stella is doing well
Back to your original source of concern - the fluid swellings in her legs
I learned that the edema (swelling) in the legs that Stella had was probably due to a protein imbalance that occurs
A protein imbalance can affect the lymphatic system ( responsible for fluid drainage )
Is there a direct link between the Potomac and the protein imbalance, or is it just an unhappy coincidence ( Stella eating too much ).
You have said that you have had a lot of rain this year - could that have lead to there being extra protein in the grass ( more rain = richer grass ) and Stella's imbalance is due to her eating more protein than usual, and also the extra rain has also increased the insect population ( you mention the mayfly that lives in water ) thereby increasing her chances of getting Potomac .
Many of us don't have a problem with Potomac - but all of our horses eat protein - normally if the grass is rich we are all looking for laminitis so limit the amount they can eat, but there are other sources of protein.