"Classic" scheme uses your system theme font colour, background and highlighting. "Kate" scheme uses black font and white background (hardcoded in scheme). Plus you have a couple of other themes out-of-the-box. Decent enough, no? If not, you may create your custom "black on white" scheme. Navigate to /usr/share/gtksourceview-3.0/styles, take existing theme as a sample (for example, Radiant-MATE.xml) and replace text foreground with #000000 and background with #FFFFFF. Nothing prevents you from copying your favourite editor scheme from Mint as well. To my knowledge, xed is a fork of pluma.
May I ask, what is the origin of these expectations? Colours are traditionally controlled by themes in gtk-based environments. Is Mint different in that respect? If you don't like theme colours, you may edit theme css directly or choose another theme. Default Ambiant and Radiant themes use grayish fonts and do not change active window's colour, but I can't say they are unreadable or have bad contrasts. If you think you found a bug, please, write a bugreport on Github or Launchpad. Pre-installed TraditionalOk and TraditionalGreen themes do change active window title and use almost black font. These ones may suite you better.
There are plenty of recent posts covering this topic, really.
This icon is from indicator-session, and it is not redundant at all. Its main purpose is to provide more convenient control over user sessions. It becomes much more clear, if your workflow requires switching between several active user sessions on one computer (for example, you share one computer with somebody or use it for teaching/educational activities). It also can be configured to show current username in panel. If you don't like it, use @lah7's advice and disable indicator-session in startup programs or uninstall it.