1. NO. Recent announcement by the current NZ gov't is in a 4 year process of removing the mortgage interest expense deduction on housing investments. What you are asking is you can claim the overseas interest expense and i'm not aware this can be done in the past ; not unless the overseas house forms 'as part of' your NZ business. Surely the incomes from the overseas house would be declared and if not then you should not have the right to any tax credit.
You should go see a tax accountant ; which would be money well spent, rather than trying to take a risk of ignorance.