2. How can I write my own syntax highlighting colorscheme? I'd like to
maybe make something like this:
http://radarzombies.org/slinky/zenburn-snapshot.png
(http://www.vim.org/scripts/script.php?script_id=415 ).

read ~/.geany/filedefs/filetypes.README

3. How can I learn about more text editing keybindings such as
up-/down-by-paragraph, indent-/dedent-current selection, go to
definition, hop between matching brackets/braces/parenthese, etc.?

indent: Tab
unindent: Shift+Tab

By the way, although I'm not familiar with GTK programming, I think
I'd read that some GTK-based editors (such as Anjuta and GEdit) use
GtkSourceview (http://gtksourceview.sourceforge.net/ ). Does geany use
Scintilla in place of that? If so, just curious, why?

check the mailinglist archive, this has been discussed before. yes it's scintilla, and that's because it has more features (or used to have more, and now it's too late to change).

I don't know about the other questions.

cheers
~luzi