I looked into a few property management companies but they want 10% of the gross rent plus other fees to renew leases and file evictions, etc.

Garnishments for my employees, for example, almost always go to my registered agent. 

I am not sure what rule you are referring to about 50% - but the quick rule of thumb that I use when talking to a seller on the phone is to take 50% of the gross income (the gross they give me without verification)  and see it as the estimated  NOI and determine the CAP rate based on the asking price.

I've had good luck getting deals in the suburbs of Detroit near the airport and have found excellent tenants - 100% airline employees.

What about Gross Rent Multiplier? 

There are also many different types of commercial leases: net leases that have base rent and tenant is responsible for their share of taxes, insurance and common area maintenance; gross leases that have all costs built into rent and a base lease year rent increase calculation is set up to deal with increased landlord expenses; and blended leases that can mix these two basic types of commercial leases.  

One investor showed me his numbers and I was surprised to see that over the last 7 years he has spent 20% of gross rent on maintenance and CapEx. 

No real structural changes have been made so public employee wages, benefits and other costs can go up requiring further revenues from taxpayers. 

Can he/she go to the closing and sign as a company employee?

I suspect that the "pay" that one gets for managing their other unit/s in a 2-4plex would just be added to their W2 employee income records.