No need for the timers. /help play.
Play also offers you the advantage that the file is buffered to memory, so that it does not have to do repeated disk access, like your filter example where you loop with $read.
Btw you forgot the -m flag in your second example, it would only message each 5 minutes now.
Also, for someone that seems to be very worried about exploits and such, you have a big exploit in your code right there. Parameters passed to a timer are evaluated twice, once when passing them to the timer command, and once when the timer triggers.
A person could have said something in a quote that makes mIRC execute commands, like | exit.
