To answer your first question.
If it hadn't been 15 years, you can often see where it may be leaking because some oil will come out with the freon and collect dirt.
Don't just take the hoses off. As Longbox55 said, you need to have it removed with reclaiming equipment. It is illegal to dump it into and pollute the atmosphere.
If you do decide to switch to R134A, remind the AC guy removing the r-12 that you know how much it is worth. Tell him you want a break on the parts to do the R134 conversion. He will be sticking your r-12 in someone else's system and charging the going rate.
You might just luck out and only need the clutch.
There is a fresh air vent under the cowl (outside, below windshield) on 67-72 AC cabs. The warm air may be coming in there. Not sure how hot it is where you are.
