Student or Learner
What is this "That in itself "? Is it the messy car inside or the cleaning out?
ex)A few years ago we purchased a brand-new camper van. Not long after we bought our camper, a friend of ours asked if her family could borrow it for house guests. We were not too interested in loaning out our spotless camper, so we declined. this happened in the fall, and we stored the camper in out backyear all that winter. In the spring my husband and I were setting it up to prepare for a trip to prepare for a trip to visit some relatives. Imagine our surprise to find that we had left cookie boxes in the camper over the winter! We'd moved and had a baby that previoous summer and fall, and cleaning out the camper had been overlooked. That in itself wouldn't have been so bad had it not been for the mice....
That in itself = the fact that they left the cookies in the camper
The cookies were not the problem.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.