One thing I would be aware of is if your tenant has a hyper allergy or similar reaction to mold, even mold remediation is not going to create an environment mold free enough for them.  

Hopefully neither of you have allergies or sensitivities which can make things worse. 

She refused to enter the house becuase of her "allergies."

Do you have animal allergies yourself? 

You can't charge for ESA's/Service animals UNLESS 1) the breed increases costs via your home insurance carrier or HOA (you can pass the increased cost difference down to your tenant, if they do not accept, you can deny); 2) the animals cause damage to the property; 3) The animal is a danger or is a health risk to other Tenants in the bldg. or to you if you live in the bldg. i.e. animal has bitten someone or allergies or animal is poisonous, etc.; 4) Animal size is too large to accommodate i.e. a horse, a 200lb. dog in a single room rental.  

If you have a guest stay with a cat, that'll end up being a miserable experience for the next guest if they have allergies.

I scheduled to have the ducts cleaned but after reading more info online , research suggested that cleaning the ducts has not proven effective in reducing smell or allergies.  

Note if tenant can get ESA approval from PetScreening.com, you cannot charge pet rent or pet deposit, but you can still charge at tenant move-out to place unit back into a pre-animal state (there are many potential tenants with pet dander allergies and if the unit was a non-animal unit move-in then at move-out it is to be once again a non-animal unit).I would pay for the block wall (shared cost with neighbor) if you do not believe it will result in other issues. 

@Matt LindseyFind a hygenist to run a test - a real mold test will test the outdoor and indoor air as that is the only true way to test since mold is everywhere - the test is to check if the mold spores indoors are different than those outdoorsNote it is also spring which news alert - people get allergies

And as @Chris Mason pointed out, most successful people on BP started with a solid foundation of six figure incomes, ultra low expenses and the extreme allergy of buying things that do not appreciate over time.