My initial infant level of knowledge would tell me it's a combination of being poorly managed (I calculated they're spending nearly 65% on expenses to only have $305K NOI on 95% occupancy at $875/m, if that's even the average rent/unit currently occupied). 

Rent for units A and C are below market and have room to increase.

You take the cash flow from 5 direct it to one.

In your case, the finance charges you are currently paying will likely be less than any construction debt you originate so I would recommend first confirming with the existing lender how to release the developable parcel and then obtain all of your approvals before proceeding. 

Just a brief overview, I am currently living in Las Vegas.

i am curious, which smartlocks are you currently using or which ones do you recommend?

If you stay at 60% or below of your homes, you can get into the 5s pretty easily with a point or two, even with credit scores as low as 680. 

While I don’t currently own any property, I’m actively looking to purchase my first one and eager to learn everything I can about real estate investment and Airbnb hosting.

In what is expected to be a continued tight housing and real estate investment market, there are a few tips and tricks that can help investors get some advantages given the current state of mortgage terms and limited housing options. 

I was thinking about an strategy of doing Cash Out Refinicing putting that money in the stock market (distributed in ETF and some "safer" stocks) to get around a 6% anual return (estimating that low because is the "standard" average, however my current stock portafolio has a cummulative return of 66% in the last 4 years).