I only just saw this thread, and it is full of fail.
When you bend, you're basically filing the string against the fret. Correspondingly, wounds strings show wear and break down, while unwound strings will show tiny polished-looking spots (go ahead, look).
The reason for this breakdown is mechanical, not chemical. This will eventually happen with or without bending, and 6 months is a very long time to have strings on a guitar, especially a daily player that you probably don't take great care of. 6 months is much longer than could reasonably be expected for strings to hold up on a daily player (lol at 'suddenly'). From now on, change your strings monthly - you'll be a better player for it.
Washing your hands or wiping the strings down endlessly will not prevent a physical breakdown like you describe, though it will keep the strings clean and sounding better longer. Salts do contribute to string oxidation and general gunk, but it's not the source of this kind of problem.
The brand of string makes no difference. Nearly all strings come from the same factories, using the same core/winding methods. The only truly different strings are DR/Elixir/Gibson/etc. coated strings. Everything else is pure branding bullshit. Before anyone throws a fit, phosphor bronze, 80/20 bronze, etc. all have legitimate differences, but not differences that realistically apply to string durability or longevity.
As for the guitar going out of tune from bending, there are a few fixes:
- Lube the nut. Take a mechanical pencil and put the graphite into the grooves of the nut (while the strings are off)
- Get better tuners. Depending on how crappy your guitar is, this may not be worth it. Also, if you ever do this, make sure the tuners you buy have screw holes that line up with your headstock's holes, or you're in for a nightmare.
- Use lighter strings. Lighter strings are easier to play, bend easier, and are less harsh on your guitar overall. An acoustic guitar has a constant string tension of up to 200 lbs trying to pull the headstock and bridge together. Old Martin's actually came with a note that anything heavier than medium gauge string would void the warranty, because bluegrass maniacs were snapping bridges off with their retardedly heavy strings.
Moral of the story is - if you break a string, it should break at the bridge 95% of the time. If your strings are breaking over the frets, they're either too old or you have a shitty fretjob, and if they're breaking at the nut, you need to have it recut.
If you're never breaking strings at all, you're not doing it right.