1. Is it possible to create a quit button for an online script?
Yes. Any clickable element along with the script.abort() function will do.
2. If they quit (using ctrl-Q or the button), will they still be directed to my dedicated end page?
If you mean your finish page as specified in the web script wizard, yes. That page is not part of the script, thus will not be skipped. Anything *in* the script, however, will be skipped.