Questions about copper tubing...
...but we had galvanized pipe leading to copper in the bathroom. I think the green tarnishing on copper pipe is not necessarily a sign of leakage, but could be due to general moisture, sweating, or humidity in the air. If you can get away with it, just leave it.
I hadn't sweated a copper pipe in my entire life when I started, but by the end of the day I had redone the shower, sink, and toilet without any leaks, so I would recommend this over compression fittings, which can crack. Sweating pipe is suprisingly easy, and goes fast once you get the hang of it.
As far as corrosion of copper pipe, I would say not in our lifetime. I also had the water main go out, which was buried 12 feet deep, but was galvanized and even IT lasted over 50 years, and was replaced with 1 inch copper, which is the standard. That's the best part about copper, is that it does not corrode like galvanized. The only problem I have now is where the copper is joined to the galvanized. These two metals will react with one another and eventually corrode, but I'm good for another few years and it's easily accessed in the crawl space, so I can fix it later if need be.
Bottom line, if you're gonna take the time to do it, do it right, sweat the copper pipe, pressure test it, and forget about it all the rest of your days.