I'm glad you're on your way again but the explanation of battery failure seems a bit sketchy.
I could understand oil in a connector could affect the charging system and other people's anecdotal evidence is of low current wiring examples but if you short circuit a battery things start smoking, melting and burning very quickly.
My money's on misdiagnosis of battery failure and a face saving exercise by the garage.
Sounds about right. I wasn't suggesting that the oil in the harness/connector was the OP's issue, or that the garage's diagnosis was correct, more that (engine) oil can cause electrical problems if it gets where it shouldn't be.