The new buyer puts down 10% as the option fee, ($50,000 fee to our buyer) in this case, and we make the rate 10% to the new buyer for the monthly payment. 

This felt like they were turning me into their money-making machine.If they’re charging $10K upfront, plus $200 per month, and taking 50% of the profits, why aren’t they putting in 50% of the work or covering 50% of the down payment?

I've seen a situation two years ago where a new landlord had trouble evicting a tenant who was constantly late on payments because the tenant argued that the late payment terms in the lease had not been honored since the start of the lease and couldnt be suddenly enforced because of this. 

I do not understand your remark as related to the DSCR loan.  2 ADUs are not allowed by f/f conventional financing but is not always enforced.Fortunately if you stay with the current tenant without a refi, by the time you pay principle it is likely the rent will have increased enough to absorb the increase mortgage payment. 

Bank pushed us to 40% down payment so also executed a $85K seller carry-back loan to cross the finish line.

Is it foolish to consider using the HELOC for the down payment?

When I tried to get it when my side of the family found out and tried tp take over reverse mortgage payments to bank because she had died it was to late so they sold the house to Hud and then other reverse mortgage companies but a relative still lives there so they can't kick them out.

A lot of them contain Balloon Payment Provisions. 

If Tenant is late in the payment of any installment of Rent or Additional Rent, orin violation of any other covenants or agreements set forth in the Lease (a “Default”) and the Defaultremains uncorrected for a period of _____ days, then Landlord may, at Landlord’s option, undertake any

If your loan is $110K then the payment on the 15 year would be $927/month at 6.15%; 30 year option at 6.625% would be $690.