So currently only 3 can be rented, but a 4th owner would like to rent out his unit so I'm trying to understand all these rules and laws to see if we can collectively change our HOA rules accordingly.

The two months of rent you will collect is not going to be worth the hassle.  

I found some collection letters including one from the IRS for $3-4,000.

At first thought, the land only seems to be the better option as maintenance seems like it would eat your cash flow up on MH repairs, as opposed to just collecting a rent check for the "land lease" essentially. 

I would collect a higher security deposit and multiple months rent if permitted by your state.

I filed last year's return but didn't report anything outside the wage earned in the states and all the taxes collected by the US were returned back to me.

In the revenue ruling, the co-owners employed an agent to manage the apartments on their behalf; the agent collected rents, paid property taxes, insurance premiums, repair and maintenance expenses, and provided the tenants with customary services, such as heat, air conditioning, trash removal, unattended parking, and maintenance of public areas.

The money will just be sitting in the bank or under your mattress collecting little to no interest that will definitely less than inflation (especially right now). 

Maybe we can collectively come up with different solutions of strategy and evaluation. 

How would I go about screening applicants and collecting payments?