I've been told Norristown is harsh on landlords compared to surrounding boroughs/townships. 

They oxygenate the urine and break the chemical bonds to destroy it.

This may sound harsh but: You should have taken this into account when you purchased the property.  

Also if that weren't the case there maybe chemicals in the new finish that may have activated the chemicals in the old finish and that is why it isn't curing.

To make a long story short, there were many calls and letters and e-mails back and forth for several months about how to make the yard “compliant.”We were told any changes to the yard had to be approved, and that the landscaping did not appear as “robust” as it once had (we had a very harsh winter here last year, and many of the bushes/plants died back—they aren’t dead though, just had to be trimmed down to the ground.

Not the only factor but a big factor.For the one I have I hire out the pool maintenance (chemicals only and filter clean) and split the cost with the tenant 50/50.

He is correct in the big, broad statement that managing 20 units for $75 each with no placement fee is a losing proposition for most people who try to go that route.It may have come across a bit harsh, but after 12 years I agree with him and would caution you against trying to implement your plan.  

I am a student, studying chemical engineering.

I know that it might harsh to give up vacation for going to school, but you can not gain without sacrifice. 

My first understanding of the pitfalls of real estate came with some harsh lessons growing up.