Okay, I think this is what you are looking for...

If you paid $2500 on the first or primary mortgage in the first month (30-year 6% fixed rate), then you would save $11,891.21 in interest (original total payments of $431,678.38 less the actual total payment of $419,787.17) and would pay the mortgage off in 348 months.

First -- and this is not the scenario you were thinking of but rather the one I was thinking of -- if you just borrowed the $2500 from a HELOC with a fixed 7%, you would have an average daily balance of $2500 plus $15 in interest for that first month. Since you are not paying any extra, your HELOC balance is going to increase over time. If you did a very simple calculation using the Future Value stream of payments, then the FV of $15 per month at 7% interest for 348 months is $16,892.51. If you then realized that this monthly interest is added to the balance every month and grows exponentially, then your balance on the HELOC is going to be $18,813.53. So you have spent $19K to save $12K.

If you could manage to have 15 days rather than 30 days of an outstanding balance, then it seems that you would pay just $7.29 per month of interest and the FV of this stream of payments is $8209.76 (7% for 348 months). However, you will be paying interest on top of interest (assuming that you do not have any extra cash and so never pay off this interest) in addition to the loan balance. So your loan balance is really more than $8209.76, it's $9406.77. Now that you've paid off the $200,000 mortgage 12 months early, you can pay off the line of credit balance. You should be able to do that fairly quickly in 8 months with a total interest cost of $243.13. So the total interest you've saved is $2241.31 ($11,891.21-9406.77-243.12).

Perhaps this is a no-cost way of saving $2241.31. You have spent 29 years making sure that your balance never went more than 15 days. But besides having to be ever vigilant over this time frame, my concerns are: 1) while it makes sense that if you only have the balance out there for 15 of the 30 days each month ($2500 loan balance for 15/30 days = $1250), the bank may have a slightly different way of calculating average daily balance (does this mean I am cynical?) and 2) it may be difficult to have less than 15 days of a balance as there may be settlement periods of 3-5 days for your direct deposit and then you may need to write checks out of your account early enough (5-10 days) to reach your creditors and avoid any late charges. But the fewer days of a balance then the less interest you would have to pay.