I understand your reasons. Since all good factions must destroy mordor to win the game, it is good to have some alternative ending. That was our reason to add the script that destroys mordor when Sauron is killed, or when the One Ring is destroyed. I see a good thing that you make an alternative script for numenor campaign.
I remember that my main concern about those game-ending scripts was to avoid the possibility to win the game too soon by using them. In order to prevent premature endings I added 2 scripts that now I'm not liking very much: the invulnerability of Sauron until you conquer certain regions (I like much more your readjusted version), and the spawn of orc armies near Mount Doom when there are enemy armies around (I plan to readjust it myself so it is only triggered in the first stages of the campaign).
You may move Aragorn army directly near Barad-dur and force a battle against the real Sauron, so you do not need to clone it. Since you know the unique name of both commanders, you can give them reinforcements as you please. Or you may remove the trait that prevents Sauron from moving and use the scripts to move it out of Barad-dur and meet Aragorn midway (iirc, in the rometw tutorial they use this kind of scripts).-If the player chooses to take the shortcut Aragorn and his stack are moved to the plains inside Mordor about halfway to Barad-Dur, a small supporting force of elves is spawned, and an impossibly large ambush force of Mordor also
-I'm thinking to make the ambush force composed of a handful of strong stacks near but not touching the two player armies and mostly commanded by generals with the 'static' trait so they don't just run off (for a few turns at least) - then have maybe two other stacks with generals who can move around and do as they please (maybe one commanded by a Sauron clone as long as real Sauron is still without the Ring and contained in his fortress - I will also disable the invincible Sauron scripts in this case)
But I have forgotten many things about scripts, I guess you know now better what can be done and what not.