So far as I can tell without being inside your head, it did, dear. 
A Faraday cage is like a vacuum chamber–impossible to reach perfection. It’s always a matter of good enough for whatever your application is.
Correct.
When it comes to EMI/RF stuff, grounding to the earth is usually not required in order for the device or system to function. It’s a common misconception, even among EEs. A lot of people, for example, think an antenna needs to be “grounded” in order for it to function, but this is rarely the case. OTOH, grounding to the earth is often required, but it is usually only done for safety purposes, to eliminate a shock hazard if there’s a fault.
Probably both (satellite signals are easy to block), but certainly the latter. Phones don’t rely only on GPS to get their location. If a phone can access any sort of network node at all (cell tower, wifi transmitter, etc.), then it can get an estimate of its position from which nodes it can see. Plus, there’s also the accelerometers: If a phone experiences no acceleration (aside from the constant acceleration of gravity), and can’t pick up any other sort of location cues, it’s going to assume that its location is remaining unchanged from the last time it did know.
We don’t need your trackers. Good question, does the service agreement for the tracker’s cellular capabilities include the roaming charges out of country?
I gather an attempt at a Faraday cage is like any other container. By analogy - you can make things pretty dark inside a thick bag, but is the light seeping in the gaps enough to work with? If you can locate yourself somewhere where the bars are minimal and your cellphone has difficulties, imagine if you cut the strength of the signal being sent/received for a device to 1/100th what it used to be. I’m not conversant on electronics, but I assume unless the metal of the antenna for the device is touching the foil, it’s fairly insulated.
I would suggest finding a dead spot (underground garage?) and while there, feed over-voltage into the device to kill the electronics. (But not so high as to burn) so it looks like the device failed.
(Sort of like the time I accidentally plugged a 5V security video camera into the 24V feed. No obvious damage, but it was dead…)
The basic stuff - The OnStar etc that cop shows love to use for tracking, or the built in GPS - that’s not exactly “surreptitious” tracking. You kind of need GPS if you want to use the map built into the car’s dispaly.
IIRC, my Tesla has basic connectivity, for things like app control and reporting location and serious problems back to Tesla. For extra data connectivity (like for watching videos while parked) there’s an additional fee.
Or wait two days for the battery to run out of juice. Then it really is just an expensive brick. It would run down quicker if you called it and left that connection live.
This is essentially correct.
It is impossible for a Faraday cage to block 100% of a signal; a certain amount will always get through. The amount that gets through primarily depends on the max signal frequency and the thickness of the cage material; Google “skin effect” for more info. And that’s for E fields. Things get more complex when you also have to consider H fields.
I dunno, but if I found one on my car, I’d hook a battery to it and mail it to Japan.
Thinking about this while driving, the only real way to conclusively clear the car of devices is a good old fashioned manual bug sweep. They use very short range wands or paddles, like in spy movies, to find hidden electronics. The obvious problem (after obtaining the use of a device & operator) is that cars are bristling with desired electronics that are going to blip the scanners just the same as the hidden ones.
‘So I guess radios don’t work on aircraft or satellites.’ Better yet, ‘If transmitters need to be grounded, how come I can see stars?’