Add this 'appendix' to your script:
; joining "your" channel - start the idle-check after 300 seconds
on me:*:join:#help: { .timeridlecheck.init 1 300 if ( $!me isop $chan ) idlecheck.init $chan }
; loop all regular users to perform the idle-check
; (using the nick-count to prevent possible kick-flood)
alias -l idlecheck.init {
var %nr = 1
while ($nick($1,%nr,r)) {
.timer 1 $calc(%nr *2) idlecheck $v1 $1
inc %nr
}
}

Edit / Note: As mIRC is starting to count the idle-duration of the channels users on YOUR join, I set the initial check straight 5 minutes after your join. There's no way to evaluate "previous" idling in your absence.