Also I guess this boils down to the question of whether or not the goal of investment properties is to own them outright at some point or just keep letting a cashflowing rental continue to pay down the mortgage for you and use that extra cash you save towards alternative investments.  

Keep hustling and remember achieving is hard but sure beats the alternative.

Alternatively, should we use the money we are throwing towards our principle to invest now?

There are many other companies now providing excellent alternatives.

You are going to have to find some alternative financing and get creative if you don't want to wait. 

My goal was to take my commissions as a real estate agent and to invest in my first fix-n-flip or to buy my first property to reside in (house hacking) but the time has come to develop a new strategy.Although I found a cheaper alternative to purchase in Long Beach, CA (250 sq ft Studio; walking distance to downtown Long Beach; 3 blocks from Pacific Ocean: $150k list price; $80 HOA; Tenant in place renting $1,200 a month), I did not qualify for conventional financing.

If you have creative alternatives, by all means use them. 

TL;DR - is a/c alternatives to window units a strong selling point to potential tenants?

In fact, it could be argued that it is significantly less risky than the alternative of purchasing a property and leaving your capital in it. 

Alternatively, there are some investors on here that are willing to help - connect with them or message them.