By keeping it they could access the cash from a sale if they absolutely had to.

Savings account.Some people recommend other options for higher return but I prefer it to be liquid and immediately accessible.

You should push forward on the late rent, that's concrete, easy to prove. 

Top/bottom duplexes are not preferred to side by side because of noise, water damage, access.

Easy solution: increase your purchase price by $5,000, and have the sellers pay $5,000 of your closing costs.

If the mechanicals of the home (plumbing, electrical, roof, and HVAC) are all centralized to a single unit, I think your home inspector stands a much better chance of spotting an issue as opposed to a multi-unit with multiple independent systems that may or may not be accessible due to tenants.As for closing costs, try to push the closing date until you have the funds available?

Do you have funds that are easily accessible in case you stumble on an offer that you can't turn down?

2- What is the difference between an Easy, Medium and Hard Turn?

I definitely see few downsides to becoming an agent, one downside just being the cost, but you have to pay for access or accreditation in any business...

It's easy to invest with friends if you've established a proper operating agreement.