Why do some characters not appear?
CharMap uses the default iOS font cascade list to render characters. Although this list includes a huge number of glyphs, some characters are not yet supported. When the app cannot render a particular character, the following symbol will appear instead:
You can still select unrenderable characters to view their details or copy and paste them to another app that may include a supported font.
It is possible to use CharMap's Font Browser to change the system font?
Unfortunately not. The Font Browser is capable of showing all of the currently installed system fonts, but it is not currently possible for apps to change the system font.