Will I ruin roasted pistachio nuts(still in shell) by freezing them? If not, any advice on how to best freeze whole in-shell roasted pistachios?
Frozen pistachio nuts should be good for about one year; as nuts (roasted or not roasted) are low in water, mostly the benefit of freezing is to protect the fat content from rancidity. You will want to make sure they are in an airtight container.
For convenience, I would shell them first, so you can use them directly when you take them out, and to save space in your freezer.
Of course, if you make them into ice cream,then you are unlikely to have any issues with them lasting very long.