Capital Gain tax paid in the year of sale on the down payment as well as in subsequent years on the principal received over the payment period.  

. -- Homeowner's  (your principal residence)b. -- Senior (over 65)c. -- Senior freeze (over 65 and income limit)d. -- Long time homeowner (ten consecutive years as your principal residence AND income limit).Do you come under any of those classifications?

That raises the initial loan principal amount due ($100K for example to $120K + closing costs) which increases the revised payment on the original asset from $530/month to $600 for example. 

The iBuyers are principals in the transaction actually going on title....not like wholesalers that usually assign the contract, they actually close.

On a HML, where one is paying interest only, since no principal is being paid down, the interest payments should stay constant until the loan payoff correct?

I have a positive cash flow of $4,222 ($527 per door average) a month after mortgage payment (including insurance and tax).I'm not including $1,056 a month in mortgage principal balance decrease with each payment.

Keep in mind that there's principal pay down on the mortgage as well.

Gift Funds:A borrower of a mortgage loan secured by a principal residence or second home may use funds received as a personal gift from an acceptable donor.

Would I be able to sell the note down the road a bit to get my principal back? 

Then, I would use the rental income to cover the HELOC payments and aggressively pay down the principal with my own contributions.