You know as well as I do that logic doesn't always work with IRS rules. And:

Complete lines 1, 2, and 7 of Table 1 by including the separate average balances of any grandfathered debt and home acquisition debt (determined by the date the debt was acquired) in your mixed-use mortgage. Don’t use the methods described earlier in this section to figure the average balance of either category. Instead, for each category, use the following method.
Figure the balance of that category of debt for each month. This is the amount of the loan proceeds allocated to that category, reduced by your principal payments on the mortgage previously applied to that category. Principal payments on a mixed-use mortgage are applied in full to each category of debt, until its balance is zero, in the following order.
First, any home equity debt not used to buy, build, or substantially improve the home.
Next, any grandfathered debt.