★.O.
I'd pump during your long drives, every time. Even on your way home - the pump is incapable of emptying your breast and even the superior suction force of the baby's mouth cannot completely empty the breast.
I'd recommend storing the milk in 4 ounce increments - and tell your sitter to give ONE bottle if baby seems hungry. Wait 30 minutes, if baby still seems hungry and nothing else works, give ONE more bottle. Spacing it out will allow your infant's stomach to realize just how much was consumed - bottles flow into the mouth and fill them up way sooner than their brains can account for - with breastfeeding they have cycles of suckling to eat and suckling to comfort, not every suckle is giving them milk when they nurse.
If she's anything like my newly 6 month old - she'd prefer to space out feeding (if allowing someone to feed her a bottle at all!) so she can be minimally satiated and wait for Mommy's return.
BOTH my children would rather wait for my return, even if it's over 6 hours. They did fine, tho I'm sure the caregiver wasn't always happy LOL