I thought the seller and buyers broker split the commission.When I obtained by broker license in 1979 all commission were 6%, and the NAR did not bother denying that this was a “set”, i.e.

If you use a HELOC, I would think about a BRRRR-ish strategy (No perfect BRRRR exists right now unless you go full off-market door knocking/postcard campaign/etc) to where you can refinance and get some of that money back, so you don't have $25k chunks of money at a 9-10% interest rate floating out there indefinitely.

I am now studying to obtain my sales associates license. 

4) If lease is ending, and the buyer decides not to purchase home for whatever reason: Unable to obtain financing through lender, doesn't want home, change in job location, etc. does the buyer or seller keep that nonrefundable deposit?

That way you obtain the liability protection of each property owned by a separate, independent entity, yet only incurr the formation and operating costs of one entity.  

Without leverage there are more passive means to obtain similar return with similar risk. - adding ADU is significant effort.

Assuming the property is equally owned 4 ways (25% each) you could have the property appraised for confirmation of value: $1.8M/4 = $450k/share x 3 = $1.35M could be obtained as a cash out refinance, potentially utilizing a healthy portion of the STR or projected STR income to qualify. 

The builder is unwilling to take the responsibilities of the damage as the warranty period has expired and mentioned that they obtained all approvals including 3rd party inspection before closing.

The benefits to using available cash to fund multiples investments as opposed to paying all cash for one investment seem pretty self explanatory, but the largest benefits...especially in real estate...are likely the ability the spread risk across multiple investments and the ability to make multiple investments in a shorter period of time, allowing your cash flow, equity, and ultimately potentially your net worth to increase at a faster pace.To answer your question about traditional financing directly, yes you can obtain a traditional mortgage and put a large cash down payment down, essentially financing 'part' of the purchase price.

You can meet flippers at meetups or try to obtain a list of people who have purchased houses for cash in your markets, then call them and see if they are interested in more.