Can I (How do I) allocate the mortgage interest expense for my second home (passive rental part of the year) on Schedule A (line 8a) and Schedule E (line 12)?

That depends on whether you used the second home personally when it was not occupied, or if it was available to rent, but just wasn't rented part of the year. 

If the rental was available all year, but just wasn't rented all year, and you did not use it personally, all of the mortgage interest goes on Schedule E.

When you are asked about the rental use days, enter the actual days it was occupied as rental days and 0 for personal use days.

If you used it as a vacation home or your primary home part of the year, just answer the questions about rental use and personal use days in the rental section, and TurboTax will divide up the mortgage interest for you.

In both cases, enter all of the mortgage interest for the year in the rental schedule and do not enter it under Deductions & Credits. If part of it should go to Schedule A, TurboTax will put the personal amount on Schedule A for you.