Why don't electrons in parallel battery circuits repel?
The individual electrons do repel each other, yes. But the overall flow of current in a wire doesn't really get repelled like that. The flow of electrons will follow the electric field, generally in the direction the field becomes more positive. In this case, the important factor is the voltage on the cells. Without a load, whichever cell has a lower voltage will see current flowing through it backwards, and this can be a problem if the battery isn't designed to handle it.

Connecting batteries in parallel is commonly done in large battery packs.  Since it is possible for cells to fail shorted, battery packs built with parallel cells usually include some sort of per-cell fusing.  Here is a picture of cells wire-bonded in a battery pack, similar to what's used in Tesla cars (picture from https://chargedevs.com/features/a-closer-look-at-wire-bonding/):



In this case, the wire bonds act as fuses and will melt to disconnect a shorted cell.