I have not resorted to a letter, always get the cosigner's phone contact and do a CC and BG check on them or you are not screening properly.

On the other hand, I have that fear in the back of my mind that something will go wrong and I won't have the experience to properly handle it. 

You can't go wrong with buy and hold in any market conditions as long as it cash flows and you do proper tenant screening when filling the units. 

Some people are just not cut out for properly screening and managing tenants. 

No 401k and IRA funds are just way easier to keep track of when invested in stocks, not to mention if you are doing RE properly you should have a minimal tax burden, so why would you use tax advantaged money to invest in it, assuming you want a diversified portfolio.

That way, they have first hand knowledge of what needs to be done to properly manage their investments before putting it in the hands of a third party.

Regarding the lot, if you plan to lease it out then yes you'll be responsible for making sure the connections (i.e. water, electricity, etc) are properly working and doing the upkeep.

Are mobile homes and manufactured homes a good idea to buy and rent out to others?  

@Andy Wilson Forgot to add to my post to make sure you screen the tenants properly.

Perhaps I'll take a more cautious approach to people who have a lot of debt even if they are managing it properly.2) Did you do anything to help them or check in with them genuinely about the impact COVID-19 had on them?