The return 0 statements at the end of some functions seem superflous, but they are not, because the previos command (even when run inside an if-clause) set the return value to non-zero, which is not what we want.

function _adjust_mtime_for_postfix_maincf
{
  if [[ $(( $(date '+%s') - $(stat -c '%Y' '/etc/postfix/main.cf') )) -lt 2 ]]
  then
    touch -d '2 seconds ago' /etc/postfix/main.cf
  fi
  return 0
}

If touch fails for some reason, why would we want to return success (0)?

The return 0 statements at the end of some functions seem superflous, but they are not, because the previos command (even when run inside an if-clause) set the return value to non-zero, which is not what we want.

function _adjust_mtime_for_postfix_maincf
{
  if [[ $(( $(date '+%s') - $(stat -c '%Y' '/etc/postfix/main.cf') )) -lt 2 ]]
  then
    touch -d '2 seconds ago' /etc/postfix/main.cf
  fi
  return 0
}

If touch fails for some reason, why would we want to return success (0)?

It's not the touch - it's the condition in the if-clause that, when the if evaluated to false, would implicitly set the return value to false, i.e. 1, that I was worried about.

Looking at it again, you're right about the if-is-true case though. So a return ${?} after the touch should also be added.

when the if evaluated to false, would implicitly set the return value to false, i.e. 1, that I was worried about.

I think you are wrong, that's not the case:

if [ 1 -eq 0 ]; then
        echo "this should never be shown"
fi
echo $? # returns 0

or

if false; then
        echo "this should never be shown"
fi
echo $? # returns 0

You are indeed correct, there is a difference between

set -eE

function x() {
  if false && :; then :; fi
}

x
echo "this will show up"

and

set -eE

function x() {
  false && :
}

x
echo "this wont show up"

I will correct my error and remove unnecessary return 0 statements.