i just did this for simple fun, and this is what i did with your snippet for my bot:
i removed the clearaway, and i added the ACTION event, because sometimes people will /me brb and then just type HELLO when they are back, so.. as soon someone talks, and it was afk the script sees them as "back". and i saved their "brb" reason

[11:59:35] * @xplo brb in a moment
[11:59:35] <@Ubot[02]> (BRB): xplo is now Away: brb in a moment
[11:59:39] <@xplo> Hello!
[11:59:40] <@Ubot[02]> (BRB): xplo is now Back, you were gone for 4secs :: brb in a moment

on $*:TEXT:/^(brb|gone|afk)/Si:#:{
  if (!$readini(idle.ini, $nick, time)) {
    set %away. $+ $nick $1-
    writeini idle.ini $nick time $ctime
    msg $chan (BRB): $nick is now Away: $1-
  }
}

on *:text:*:#:{
  if ($readini(idle.ini, $nick, time) >= 1) {
    msg $chan (BRB): $nick is now Back, you were gone for $duration($calc($ctime -$readini(idle.ini, $nick, time) )) :: $($+(%,away.,$nick),2)
    unset %away. $+ $nick
    remini idle.ini $nick
  }
}
On *:TEXT:*:#: {
  if (%away isin $1-) { msg $chan Sorry $nick but there marked as brb at this time, they will say back when they return. }
}

on $*:action:/^(brb|gone|afk)/Si:#:{
  if (!$readini(idle.ini, $nick, time)) {
    set %away. $+ $nick $1-
    writeini idle.ini $nick time $ctime
    msg $chan (BRB): $nick is now Away: $1-
  }
}

on *:action:*:#:{
  if ($readini(idle.ini, $nick, time) >= 1) {
    msg $chan (BRB): $nick is now Back, you were gone for $duration($calc($ctime -$readini(idle.ini, $nick, time) )) :: $($+(%,away.,$nick),2)
    unset %away. $+ $nick
    remini idle.ini $nick
  }
}
On *:action:*:#: {
  if (%away isin $1-) { msg $chan Sorry $nick but there marked as brb at this time, they will say back when they return. }
}

Your base code is nice, it gave me an idea, and i liked it.
+like
8/10