The cells which are in parallel will auto-balance when inserted. It's true that putting an empty cell and a full cell in, can indeed lead to a unextinguishble fire due to thermal runaway. But from the factory, cells will be very close already. They will auto balance once inserted (the parallell cells). Every single charger in existence will then balance the series cells actively on every charge cycle. So "terrible idea"? No, but maybe something to leave to the bike shop. Still better than throwing away the pack. And maybe fuses between every cell would help for protection against