Been listening to bigger pockets for about a month now (literally everyday, at every moment I have spare time LoL) and am learning a lot.

(For example, IF I was able to purchase the first home (multi-family) outright, paid-in-full, then use a HELOC to pay for the down payment and closing costs for the second home, would that be a smarter approach?)

It won't change the payment unless you refinance.

Co-living is how I went from renting a room, Delivering Dominos, no savings, and not being able to qualify for a loan to financial freedom & a $2M net worth in 15 months.  

I'm looking at monthly cost to seller vs. anticipated rental income, purchase price relevant to the ARV of the area and interest rate, balloon, etc.

So, you can do a land contract for 12 months, which should allow lender to credit you full value of the land vs the purchase price, but the loan will pay off the land contract, so the lender maintains first lien position.Can't help you with the rest of your question, but you should be able to search for "CPA" here on BP to find experts to reach out to.

But there's something else that many people don't realize: these loans  "constant recast" during the interest-only period.For example, if I make an extra $400 payment one month, my interest-only payment will decrease the following month.

I originally wanted to buy into a vacation rental market but this last year has been crazy and we seemed to have been priced out.  

HostGPO charges you a fixed monthly membership fee.

Because instead of collecting one rent check per month, you’re collecting multiple payments from different tenants.