I would think it would be better to pocket carry something in your situation than trying to put on a holster in your car, or worse yet, taking it off body into a location and putting it on in their bathroom.
Unless you are going into a highly desirable robbery target (gas station, jewelry store, liquor store, etc.), your odds of being assaulted are highest while you are entering or exiting the store. If you are waiting until you enter the building to put it on, you will not have access to your gun during one of the times you are at your greatest risk.
Furthermore, every public restroom facility I have ever been in is far from private. They receive heavy foot traffic, and bathroom stalls often have a large gaps between partitions. Not to mention the curious child that comes in and looks through the gaps and tells his daddy in a loud voice "He has a gun!". Because of your lack of certain privacy, there will be a self-imposed pressure to make sure you put on your carry rig quickly, and quick gun handling can lead to safety issues.
As far as the car goes, the pressure to put your gun on quickly still exists, and to top it off, you are going to be in an awkward body position, further amplifying your risk of something going wrong. One of the most common times for an AD is while re-holstering a firearm. In the car, your position and hurriedness greatly increase this possibility.
The only viable option, IMO, is to have a good, simple pocket carry solution and be as proficient with it as you are with your primary. I realize it is less ideal to carry a J frame than to carry a higher capacity semi-auto, but the J Frame is fully capable of getting the job done, and to me at least, is the absolute perfect fit for what you are describing. Pocket semi-autos are also good solutions.
