.; what are their occupations, what is their income, how long have they been there, have they caused any property damage, how well do they seem to maintain their space, what proof does the seller have that the tenants have been paying on-time, etc.?)...

However, if you property manager is willing to cause tension in their relationship with you over a $50 late fee it might be time to find a new property manager. 

Every single one of these moving pieces will cause more work for you, and every single one is a potential point of failure in your venture.

Maybe self employed borrowers have ups and downs year to year and that caused problems in underwriting.

In addition, how do we get to that income.I can see that we have the Monthly Rent Income and then the Monthly Note Income, how do we get to receive the note income of 12% / year, cause that seems to be quite important in addition to the rent.

.- they moved out but did not inform their agent or the insurance company- they rented the structure to a tenant who caused a fire lossThere is not an absolute answer.  

Other pets, like a turtle, that are predominantly in a cage and won't feasibly cause damage are fine.

In that situation you can be evicted without cause, meaning the tenant did nothing wrong.

They made the errors that caused the damage.

It put it behind and caused a lot of extra work.