Although there COULD BE some picayune differences, I trust EAC rather explicitly. If it says you've gotten 100%, then you've gotten so close to 100% that it'd be VERY hard to tell the difference.
In my experience, the MP3's created by LAME using the recommended settings that EAC wants to use are very good also. However, certainly it's possible to get BETTER MP3's, if you rip to wave and then use some other program to create the MP3's.
But your drive has (almost) NOTHING to do with the end quality of the MP3 if EAC says that there were no errors. Fear not!
Also, does it effect the quality of the extraction or encoding if I'm using the computer, while ripping/encoding, to surf the internet or to word process?
Not at all, unless you're causing frequent buffer underruns. And even then, not really. Surf away!