Both directories are in your path, and many distros have /usr/bin as a symlink to /bin or vice versa, so it almost certainly doesn’t matter. Also, the location of the shell has no bearing on which shell is used when you log in as root. That’s determined by the login shell field for the root user in /etc/passwd, which is most likely set to bash or /bin/sh (which is most likely a symlink to bash) by default.

The word you’re looking for is “copula”, and it’s actually 「です」(desu)

It’s actually 「は」, and… kind of. It marks the topic, which is sort of the thing the conversation is generally about, which typically is the subject of each sentence, but not necessarily. It’s kinda hard to explain it well since it doesn’t really map cleanly onto any grammatical feature in english.