If I truly got along well with my ILs, after having spent a lot of time with them, I'd consider it on a medium term basis, especially if we only lived together on weekends. I'd set a specific deadline, and also make sure the terms were very clear (would be be contributing anything in rent, what would we pay toward utilities, would there be any expectations of additional chores around the house, any areas off limits, etc.) just to help make sure everyone was on the same page. And I would set a deadline to move out. If you rent your place for a one year term, the end of the lease might make sense if you would move back to the same house. One year with mom and dad to help get your footing again, but then you move out so it doesn't become a forever thing (unless you all find it works perfectly and you are actively contributing to the household so that it is a true win for everyone).
As for the cars, I would sell the truck if that's okay with FIL. The Civic is going to be so much cheaper to operate. You mentioned going to one car. If you are considering that, of the choices I would go with the Civic. If FIL doesn't want you to sell the truck, perhaps you could agree to split the proceeds with him if you do all the work of the sale, or something like that.