"if leaving a gun overnight with someone is not a 'transfer', then why does a Gunsmith have to log the 'acquisition' in, and the 'disposition' out, if he keeps a gun overnight? "
The gunsmith is in business. The gunsmith has a license issued by the regulating authority. The bound book is required by said regulating authority. Follow the regs or lose your license. Surely you know this.
Surely you also know that us common non-gunsmith folk don't have an FFL and aren't bound by those ATF regs.
My uncle borrowed a Marlin .444 from his brother in law and hunted with it for 7 years. When his BIL wanted it back it was returned. There isn't a court in the country that would find this to be anything other than a loan. That's just one example.