As Neil Young sings - "Rust Never Sleeps".
I'm certainly no expert, but...
Crevice corrosion is genrally associated with stainless steel and is often caused by oxygen starvation. "Stainless" Steel needs free oxygen movement around it to enable it to corrode evenly. If a portion is located in a dead space (such as where chainplates pass through the deck and are then sealed) you will get concentrated crevice corrosion in that area. Throw a little stagnant seawater into the formula and the chainplate WILL fail right at that hidden point.
Conventional wisdom tells us to replace the chainplates every other time you replace the standing rigging just for this reason.
Nylock nuts should be avoided for this reason, as well.
We ran into a friend in Port Suez who was coaxing his way north under a jury rig. When asked what caused his dismasting - he showed me half of a chainplate and said "look at it - no wonder it broke - they must have used the wrong welding rod when they made this thing - it's only tewnty years old!" I asked when was the last time he'd checked them and he replied "I NEVER HAD TO!" Duh.
To answer your questions, in my opinion:
You'll never be able to restore corroded parts to original strength.
Periodic polishing will retard / prevent further corrosion.
Precautions against allowing this to happen in the future would be to perform periodic inspections, isolate dis-similar metals, eliminate any stray current that may be getting to the hardware and consider using alternate metals in future applications, such as bronze or titanium.
I reckon UNOBTAINIUM would be the best metal for most marine applications - but you'll find unobtainium is nearly impossible to find!