I highly suggest not leaving a gun in your vehicle full time. If it were to be broken into, not only will you have lost a gun but a criminal will now have a firearm. Not to mention in many cases a firearm that is titled to you. In many states there is no gun registration, however we all have to fill out paperwork on them when we buy them legally and if used in a crime they will eventually track the firearm to you and have questions. Leaving it in a car in some states could be seen as neglegence. I used to leave my Glock in my truck all the time and one year it got broken into and the guy took my gun. They caught him shortly after and I got back my gun. But it could have been a lot worse. He could have killed someone with my gun, or even decided to come in my house now that I'd armed him. I was young a stupid but I learned my lesson. There's an awful feeling about someone else having YOUR gun.
As for ammo, I just store it in military ammo cans. The ones with rubber gaskets keep it moisture free. I just put the ammo in a crown royal bag to keep it away from the metal walls of the can so that if it get's too hot it won't cook a round off. But it's gotta get REALLY hot to do that so the CR bag is really overkill. Plus I toss in a few of the silicone packets that you see in brand new bags sometimes to keep moisture down. FME normal car heat (100 to 180) doesn't really hurt ammo. Moisture is what will kill it. But most modern ammo is sealed so well it takes a lot.
As far as mag spring set, I've read a lot about the metalurgy of mag springs and from what I've found a spring will take a set when you push it beyond it's load capacity. Much like our knives, if you bend them within spec they return to true. You bend them further, they take a set or snap. A quality mag spring will be made with it's load capacity in mind. So that when full loaded it's still beneath it's max load capacity. In this case, leaving it loaded, unloading, loading it, none of that will harm it. If it's not designed properly and loading it fully exceeds it's load capacity, it'll take a set no matter what you do. So the best thing to do is test your mags often. If one has taken a set and introduces feeding problems, change the spring and start over. The ones that don't take a set, are working within their load capacity and shouldn't ever really give you a problem unless damaged. Older single stacks are bad about taking a set becuase a lot of them just have springs in them. Not really calibrated with a load in mind. Most modern mags will let you know within a year or two if they're gonna become a problem.