Welcome to the forum, tk421.
1) The data is stored in this folders: www.rainlendar.net/cms/index.php?option=com_easyfaq&Itemid=26#faq1
2) It's possible to replace the bitmaps with fonts, but you will need a lot of work to keep the same aspect. The Shadow4 skin was made using the free font Fusi. The opaque on mouseover is a general setting, works with any skin.
3) Search for the \skin folder in #1
4) There is a Widget file by the same author of Rainlendar. It has a moonphase widget. www.rainlendar.net/cms/index.php?option=com_kunena&Itemid=42&func=view&catid=6&id=6164
or direct link: www.rainlendar.net/download/Shadow4-Widgets.r2skin-addon