Getting rid of mildew is as simple as spraying some bleach water with an insecticide sprayer.  

Can I remove the mold portions, wipe the remaining with bleach and seal with Kilz Sealer; then wait a few days and get the air tested before patching the drywall back up?

He told me he would place a few fans under the house, and spray with bleach etc He has experience with this type of issue.

The run 4 vaporizers filled with bleach for a week or 2 to kill anything that might have spawned down there, then dry it out again.

Thirdly, tear out all the effected drywall/insulation and wash the studs with bleach and water.Finally, if the first test showed a health issue then retest the property.

As for your tenant, the cost is on them but it is likely limited to a new carpet and some bleach.

The beat bang for your conversion buck in grey water is seperating the was shing machine(s), but unless renting in a hipster area where this would be a feature, do you really expect a tenant to watch what type of soap they use, or flip the three way valve to divert to the sewer when using bleach on thier whites?

Go over, spray some bleach water/hydrogen peroxide (or, if you wish, spend extra money on "mold remover spray" from Lowes/Home Depot) and take care of this.It REALLY is time that landlords stop worrying so much about mildew.Gail

My husband attempted to clean with bleach and they freaked out stating that they wanted him to leave it for evidence... 

You'll probably save money paying a handyman or some young person to remove the old carpet and pad.... and we'll go ahead and assume there are pet staining... so make sure you have someone get in there and bleach the floor and paint it with Killz or a product that is anti-microbial.