Would it be still possible to buy real estate through conventional methods without W2s?

I don't think there was ever talk of wanting a single person specifically, the landlord mentioned charging more for multiple people living down there. 

If you think you will go FHA, Conventional, FHA 203k, etc. and then Quit Claim the property, to a LLC, or a Land Trust you run the risk of the lender discovering a Title Transfer occurred and activating the "Acceleration Clause" or "Due on Sale Clause" that requires the loan to be paid in full, within 'x' number of days.

He is willing to finance the property if I put down 12%, he will charge me a fixed 3.5% interest rate for 5 years with a balloon payment at the end of the 5 years.

To buy the condo in Tahoe, of course, I had to get a conventional mortgage.

However, if you can get $1100 SFL for $75K, I think that's still a good deal.Also, one thing i noticed that these calculators are missing are if you are using a PM (which it looks like you are), they charge a placement fee, which should def be incorporated into the numbers. 

They may charge you a higher interest rate than a bank and are usually short term lenders so you might have to refinance it after 6 or 12 months so you can find a long term lender or just sell it. 

Should I take out a HELOC or HEL on my primary residence, and use the funds from that to make a down payment on additional properties and then obtain a conventional mortgage on them?

In today's current lending environment, no matter where the asset is, a line of credit on an investment property, through a conventional lender, is almost near impossible to obtain. 

I was told that my two ongoing flip projects (under my LLC with a partner) would be included as debt in my DTI ratio and therefore wouldn’t qualify for my jumbo conventional cash out refi option (seeking $540k note).