For HELOC loans where the interest rate is never locked, should the "rate set date" used to determine rate spread be the last date the interest rate was adjusted before the final disposition?

If your HELOCs are based on a variable rate, then likely that would be the day of closing.

2. Date the interest rate was set. The relevant date to use to determine the average prime offer rate for a comparable transaction is the date on which the loan's interest rate was set by the financial institution for the final time before closing. If an interest rate is set pursuant to a “lock-in” agreement between the lender and the borrower, then the date on which the agreement fixes the interest rate is the date the rate was set. If a rate is re-set after a lock-in agreement is executed (for example, because the borrower exercises a float-down option or the agreement expires), then the relevant date is the date the rate is re-set for the final time before closing. If no lock-in agreement is executed, then the relevant date is the date on which the institution sets the rate for the final time before closing.