upper or lower dont bother
your alias can be at this style

alias AddNick {
  var %r $read(acceptednicks.txt,$address($1,2))
  if (!%r) .write AcceptedNicks.txt $address($1,2)
  echo -a 7 $+ $1 is $iif(%r,already on,now added to) Accepted Nicks.
}
alias DelNick {
  var %r $read(acceptednicks.txt,$address($1,2))
  if (%r) .write -dl $readn AcceptedNicks.txt
  echo -a 7 $+ $1 is $iif(%r,deleted from,not in) Accepted Nicks.
}

and about the alias shownicks u can use loadbuf

alias ShowNicks {
  if (!$window(@AcceptedNicks)) window -znkg1 @AcceptedNicks
  loadbuf @AcceptedNicks acceptednicks.txt
}

about on open event u can use $wildsite instead of $address($nick,2)
and use $nick(#,$nick,ov) instead of ($nick isop #chan) || ($nick ison #chan) || ($nick isvoice #chan)

finaly just a sugection about your menu on/off

menu channel,menubar {
  PM Blocker
  .Set $iif(%PmBlocker == on,OFF,ON):set %PmBlocker $iif(%PmBlocker == on,off,on) | echo -a 7PM Blocker is now %PmBlocker $+ .
}