Using higher octane fuel won’t hurt you motor. I’m glad the pinging is gone. However I’d advise against lugging the motor, because that will hurt your motor.
Shifting to a higher gear and keeping the rev’s down is a good way to max your mpg, but with the small motor in your car you have to balance the mpg without lugging the motor. Go to the higher gear and keep the rev’s low on downgrades or flat terrain, but not on hills.
Your motor takes higher rpm to make sufficient torque to cope with inclines. So on hills let it rev to about 3,200 rpm then shift to a lower gear and keep the rev’s at 2,500 or higher as long as you are going up hill. My guess is you’ll use less fuel going uphill at 3,000 than lugging up the same hill at 2,000. At the higher rpm your motor will he happier and last longer. Lugging the motor puts a lot of stress on the bearings, and crankshaft.
Going up hill at a few more rpms won’t do much damage to your mpg, but will avoid damage to your motor.