Telling a guest that they are paying less to put up with an inconvenience will just highlight the inconvenience and does little to remedy the annoyance.

My main concession would be the cost to remediate mold issues.

Almost if not all failed portions of your inspection report need to be remedied completely by the seller before closing.

In either case, the remedy to market knowledge is talking to more investors and RE professionals (PMs, agents, even contractors can be a great source of info) in your market as well as reading whatever information you can on the areas.

Learn about remediation of cat piss,.. trust me, it’s a horrible experience but totally doable.

If not, what should be done to remedy?  

I would think you would need an engineer to plan out the flood zone remediation method in order to get building permits approved. 

If it's not in the specified condition, either refuse to close or do a hold back of funds sufficient to remedy the situation.Careful with the electrified fence. 

your remedies are state specific. if WA is an attorney state, consult with the atty.

Learned a lot about environmental issues and soil remediation.