I disagree. If the dev hadn't forgotten the "/var/run/mpd" directory in the pkgbuild, everything would have meshed together smoothly. The pkgbuild should be fixed, not the install script having to catch every posible error.

About the other issues:
1] Let the installer see stderr but not stdout
. ex, change 'userdel mpd &>/dev/null' to 'userdel mpd 1>/dev/null'
2] simplify stdout/stderr redirect in getent
. ex, change 'getent passwd mpd >/dev/null 2>&1' to 'getent passwd mpd &>/dev/null'
3] Doesn't hurt to check with getent before adding groups/users. Also doesn't hurt to check/enforce {SYS_UID_MIN,SYS_UID_MAX} (add "-r")
. 'getent passwd "mpd" &>/dev/null || useradd -r -u 45 -g mpd -d /var/lib/mpd -s /bin/true mpd 1>/dev/null'
. 'getent group mpd &>/dev/null || groupadd -r -g 45 mpd 1>/dev/null'
. 'groups mpd | grep "\baudio\b" &>/dev/null || gpasswd -a mpd audio 1>/dev/null'
4] See #18780, but post_remove should follow pacman conventions
. '[ -f etc/mpd.conf ] && mv etc/mpd.conf etc/mpd.conf.pacsave 1>/dev/null'

5] Avoid "jacking" permissions, lol, and let user know of errors
. 'for dir in /var/{lib,log,run}/mpd; do chown -R mpd:mpd "$dir" 1>/dev/null; done'

You might also want to check that the mpd group/user exists during the post_upgrade too, perhaps by moving all the (new) getent stuff to post_upgrade(), and calling post_upgrade in post_install()

!Not Tested!