High potassium can also cause "rotten egg" smell.
You might get good info from calling up the local "water treatment" people. I went to the local water/testing lab (ask a real estate agent who they use for water testing). I paid
around $200 for the full analysis. I wanted to MAKE SURE of the condition of my water supply.
Suich labs are independent, licesned by the state, and sell no equipment. I got the whole assay of almost 200 different components, including fertilizer, chemical contaminants, nuclear, etc.
All of the local water treatment companies will likely get different results. It would be okay for a quick test (you can get similar testing kits from a local swimming pool supply).
You have a lot of good ideas here. Let us know what you find out as you move along.