There's no internal spell checker in C5 that i know of, but if you use a web browser such as Firefox, or Chrome they have built in spell checkers. That way you are able to spell check everywhere on the web. :)
Yes but it's not something that is set up to function "out of the box", plus it is only limited to TinyMCE instances and not other inputs and textareas. Using a spell checker within your browser is not limited to the C5 install and the WYSIWYG's there in. It also doesn't required any user interaction in order to work.