I don't think making an "unending delay" for MCEs is worthwhile. I am, however, already planning on using a 2 minute delay for seeker powerups in some of my levels.

For the delay parameters, I may go with seconds only instead of minutes and seconds so it would be more intuitive (3 minutes and 63 seconds is a little awkward) and so events can use a slightly higher maximum respawn delay (255 seconds versus 243).