Is it correct that someone can own even just 1% of a "qualified home" that is his secondary residence, and that person would be entitled to a deduction on all of the interest for any part of the secured mortgage that he paid? Publication 936 sets no rule - and makes no mention - of the percentage of ownership. Instead, all of the rules seem to be set around secured mortgage, the amounts of interest, the type of loan, and whether it is a primary or secondary residence.
One situation where this could happen is a husband divorces and at the time of settlement gives 99% of the family home's equity to the wife. He is left with 1% ownership and agrees to pay the mortgage in full. In this case the family home is still his second residence and he is still responsible for the mortgage, which is secured by the home. Husband might be better off just paying an alimony to the wife, but I'm not focused on optimizing a divorce. I'm focused on whether the home mortgage interest deduction rules allow this kind of situation where a low-equity owner is able to fully deduct all mortgage interest he pays, regardless of the percent he owns.