Not deliberately or negligently destroy, deface, damage, impair or remove any part of the premises or permit any person to do so; and shall not engage in any illegal conduct on the premises; and Act and require other invited guests on the premises, to act in a manner that will not disturb the neighbors’ peaceful enjoyment of the premises.

Thank you for the kind words @Kim YounkinFor the copy and paste impaired here is the link to The Ultimate Guide to Grading Cleveland Neighborhoods.

HUD requires reasonable accommodations to any reasonable difficulty or impairment that a tenant may have, it is absolutely not restricted or limited under the ADA or limited to only physical impairments.

., a physical or mental impairment that substantially limits one or more major life activities?

In your case, I would justify the intra-lease charge by pointing to the impaired usage of the property for the other tenants and the pet addendum terms calling for immediate cleanup of animal waste.  

., a physical or mental impairment that substantially limits one or more major life activities? 

Examples include providing a parking space for a movement-impaired tenant and accepting a guide or service animal in an otherwise “no pets” building. ?

This is defined by any "disability" that impairs normal functioning and that the person must take medication or treatment for.

It may be that this owner couldn't qualify for a conventional mortgage or didn't want to make payments (the principal and interest just accrue), or planned to stay in it the rest of their lives.I don't think this would impair any purchasing opportunity.

As shown by your analysis, the effect of time on the cashflow has impaired the IRR/Rate or Return by 3.58% (i.e. 10.01% - 6.43%).