Yes, but the plan9 text editor/shell/IDE is called Acme: http://doc.cat-v.org/plan_9/4th_edition/papers/acme/

yes, but the shell isnt responsible for the terminal or the editing capabilities on the rio windows. unlike unix, plan9 has no ttys or cursor addressing or no ansi graphics.

everything is just (UTF-8) text.

the window system and acme allow you to edit whats on the screen. and theres a mechanism called the plumber that lets you execute various action depending on selected text.

see:

http://plan9.bell-labs.com/sys/doc/plumb.html

and

http://doc.cat-v.org/plan_9/3rd_edition/rio/