I haven't read the entire thread, but yes, on ALL interest-only mortgages if you pay down any principal then the remaining monthly interest-only minimum payments will be reduced.
Less principal = less interest accrues each month, and it IS an interest-ONLY loan. Calculating monthly interest is easy. Take the principal balance multiplied by the interest rate in percentage (i.e. 5.75% = 0.0575) divided by 12. For example, ($450,000*0.0575)/12 = $2,156.25 monthly accrued interest. Reduce principal balance by $50k and you get ($400,000*0.0575)/12 = $1,916.67
Be advised though, all interest-only loans allow interest-only payments for just the first 10 years (120 months). Then they reamortize into full principal and interest payments for the remaining 20 years (240 months), depending on the remaining principal at the 120 month mark.
I sincerely doubt that there is any lender out there who allows a borrower to lock in a rate 9 months (270 days!) in advance, especially when most locks are from 20-45 days. There are some who do allow 6 month (180 day) locks, but doing so is generally quite expensive (lock fees are always calculated as a percentage of the loan amount) and I usually advise against it. No one has a crystal ball, and you might pay an expensive lock fee up-front only to find that rates are similar 6 months down the road.