Patti- Ok, I do not know how many acres your pond is, but if you have 3-4 grass carp/acre then you would be fine.
Were you consulted about the addition of the culvert under the road on to your land? In answer to your question about fertilizer runoff from your neighbors into your pond being a problem. YES IT IS A PROBLEM. The addition of added nutrients will accelerate the aging of the pond and you will have more algae and plant growth which you may not want. Regarding whether the fish are edible in the pond. Yes, they should be fine to consume. You should not glow in the dark after eating a few bass or perch. You need not worry about the freshwater runoff into the pond that I think your husband had a concern with. The runoff water is not a concern but the extra nutrient load from the neighbors yards is a concern. City folks typically over fertilize their yards to keep it nice and green and do not understand or care about runoff. You may have been able to have stopped the developer from installing the culvert had you caught it in time because his runoff was and is impacting your property.

Several years ago I had a lady that was determined to drain a swamp because she wanted to develop the land and sell " mini ranches " of 5 acres. So she hired a dozer operator to channelize the swamp and increase the drainage. He stuck the dozer a time or two until I realized what she was doing. Her increased water from the drainage ditch would increase the erosion on my property causing me harm. I traced her water source all the way to the formation of a creek which flowed for several miles until it entered the Kiamichi River East of Antlers,Oklahoma. The Kiamichi River is a navigable waterway. All drainages which enter a navigable waterway that might impact the waterway are controlled by the US Army Corps of Engineers. Long story short, an inspector from the Corps contacted me on his cell and I directed him to the subject site. He was able to confirm the drainage ultimately entered the Kiamichi River.Hehad a chat with the lady neighbor and her drainage project stopped dead in its tracks.

I hope I answered all of your questions, if not let me know.