1. You wouldn't be able to see a skip yourself - maybe some software could analyse the MP3s and possibly find them, but it would be a slow process.
2 & 3. They may cause a skip, squeak or other audible error. I don't exactly understand what the errors are, but they're both bad.
My suggestion is to set grip to use full paranoia - this will use cdparanoia, which does everything it possibly can to avoid errors (e.g. re-reads sectors 25 times if necessary, slows down ripping etc.)
If you use full paranoia, then you're really doing all you can to prevent errors.