UPDATE:
Waited 24-30 hours for the Battery Tender to fully charge the battery.
Reinstalled battery into FR-S.
Started up just fine. No stuttering before it fires up.
Then I made a short trip to the grocery store, another short trip to the car wash, and then a longer trip to another grocery store. Each time I started the car, it was fine.
MISSION ACCOMPLISHED!
Thanks to those in this thread who made great suggestions!