I don't see why your PIND stuff isn't working.
[Edit to add ...]
I was thinking of an Uno, but you are using a mini so maybe the pin mapping is different.
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Try writing a short sketch that explores that and nothing else - and just showing the values you get. Don't bother with interrupts for this purpose.
Then extend the short sketch bit by bit (sorry for the pun) until it nearly matches the real code.
...R