Not all trains on that line stop at Blair Atholl, that is why you are being offered a change at Pitlochry. No reason why you shouldn't do that.
In Aviemore there is a ticket office and machine with limited opening hours but you can buy a ticket on train also which will be the highest price. Buses will be cheaper.
The Tesco is 24 hours and they most likely will stock meths if that's what you use. Maybe you could check on line.
All the best with your trip.