Reposting one more time to see if anyone else finds this bothersome
Long question short: Is there any surefire way to hang onto cars while enjoying them in single-player?
Nah, unfortunately the devs of Grand Theft AUTO did not feel it was important to secure your car, even after spending 10s of 1000s on 1000s of upgrades and customizations, in a world where every mission ends in a joke where you don't get paid in game, if you wanted to get money, you had to go to an internet forum and exploit fake insider day trading.
To me, being able to keep a car was the one thing they HAD to be sure of. Even in original San Andreas on a damn PS, I could get 4 cars in a garage and somehow most of the time, they would stay. In Grand Theft AUTO, picking your own car of choice, pimping it out, turns out to be pointless. Since money is a joke, you can't even work around the issue.
Amazing world, okay story, shit dialogue, some decent comedy, overall the game is full of way too much bullshit.