Ignition key will not turn to "off" position

I bought this car from my wife's aunt and uncle in 2008 with 51,000 miles on it, but a great deal of it was in-town mileage. After about six months the ignition key started giving trouble releasing. The ignition would only turn far enough to align with "ACC" accessory position but would not turn any further to the OFF position enabling the key to come out. I left the key in the ignition and locked the doors with a small rag tossed over the steering column which hung down enough to cover the fact that the key was still in the ignition. After driving the vehicle regularly it became apparent that this was an intermittent occurrence with no real reason I was aware of for it doing this. With the vehicle out of warranty I called a locksmith and he quoted me an astronomical price to make a new key to fit the lock better and to thoroughly clean the lock mechanism and lubricate it. $880.00 was too steep for a car that I paid $1,200.00 for. I have lived with this problem for close to eight years with it being stuck in the ignition sometimes for as long as two weeks at a time and for as little as a few minutes. Each time I turn off my car it is a toss up whether or not the key will come out of the ignition properly. I have heard of people hitting their key with a mallet while it is in the ignition but with my luck the key would break off inside. For awhile having my key in a certain orientation proved to be somewhat beneficial but that was short lived after only about forty five days of continuously working. I have noticed in the past that keys to vehicles I have owned "wear down" over time. The high notches on the keys very gradually become shorter and sharper cuts become rounded edges over time. If you get another key made from one that has worn down then you have had a worn out key made. I do not know if a worn key is the problem or if the interlock that prevents the key from coming out while operating the vehicle is the problem. I have heard that the ignition key interlock (not the interlock preventing shifting from Park without depressing the brake pedal) will become an issue only if your battery is weak because full voltage is required to release the key. In my specific vehicle is there a way to find out the cause for sure. I have tested my battery using two different meters that I own and after sitting overnight in cold weather it is always at 12.97 volts. Even after two to three days of not driving it with the key stuck in the ignition in the ACC position; but I turn off all accessories and unplug my cell phone charger, the voltage is still at 12.97 volts. Of course I do not have a tester to measure the amperage of my battery, but each time I have had it tested at a reputable shop they tell me that there is nothing wrong with my battery or starting system or charging system. They have recommended replacing the ignition lock cylinder, but warn that it could end up creating a problem with starting the vehicle if the ignition does not match my vehicle. I have no chip in my key and it is a single key that opens doors, trunk and used as ignition key. I have had no issues with any door locks or the trunk lock whatsoever. Hopefully you can provide some insight on this matter.