1. That's confirmed:
With the Custom Design upgrade, you can make your blog look exactly the way you want. The Font Editor lets you customize the fonts in your theme in a just few clicks — no coding knowledge required! The CSS Editor lets you customize the stylesheets of any available theme. Some themes, like the plain but versatile Sandbox and Toolbox, are blank canvases waiting for your custom CSS, fonts and images. Please note, however, that the Custom Design Upgrade does not permit the editing of the core PHP or template files of any theme. This upgrade costs $30.00 per blog, per year. http://en.support.wordpress.com/custom-design/
Custom Design > Frequently Asked Questions > http://en.support.wordpress.com/custom-design/#frequently-asked-questions
2. Yes. Every upgrade is for one year and for one blog only. If you fail to renew it means the CSS editing you did disappears and reverts to the normall appearnce of the theme you customized.