I've played with the harad betrayer list on 500 points a couple of times and found it very fun to play against. I would suggest putting all your archers in the warband with the betrayer and give as many of them spears. Due to the army bonus all spears will be poisened as well giving you a surprising punch in combat. The 500 points list even had the betrayer on foot since his main function is to use his poison ability and spam magic. You don't want to use him in combat and on foot he is a harder target for enemy archers.
Your shooting is quite devastating getting a little over 2 wounds on sixes each turn on average. By the time on foot armies reach you, you could outnumber most armies in combat 2 to one, allowing for lot of spear support reroll failed to wound spears.
While it's true you do not really have an answer to powerfull enemy heroes like aragorn, but you can always employ the surprisingly fun tactic of having one warrior charge him and then shoot with everything else into that combat. If you're lucky you might wound him, but with so much shooting it's very likely you will kill your own warrior, cancelling any would be heroic combats.

I think all of those lists seem quite decent. Although I don't have a ton of experience with any of those factions.
The one tweak I might make is to Far Harad have it be an allied list with Serpent horde. I feel like the tribesmen with camels are pretty meh, but I see why you wouldnt want to take too many cavalry and have a tiny force, but just those regular troops in Far Harad seem overpriced and bad and since you can ally you might as well if you can eek out a better mix but still get the damage of the camels, which is gross by the way, and the Mahud king of course, who's pretty good for his points.
The list with the Betrayer looks the weakest to me but I am not really sure why, just kind of gut reaction. I guess that one feels like it has the least pieces for opponents to have to play around or worry about. You have cav, good at recon and a wraith good at dealing with monsters and big heroes but other then that you're fight 3 mostly and low defense without a big killy hero.

As to which you should take, probably the only thing that really matters is what you think you're going to be playing against and which list will do better against the other armies you'll likely see. If you have no idea what you're likely to see I'd probably lean towards Isengard, since the cross bows give you a little more variability then some of the other lists and Isengard has almost no bad match ups(really) so as long as you play well you'll probably do pretty well.

I was going to say go with Isengard or Numenor at first but could not decide between the two. Seeing this new harad/far harad list though. I think it might be the way to go with two decent heroes, cav, and alot of killing power from the guard and camels. If I were going to fight it I would bring your Isenguard as a counter to gun your heroes and cav down. Idk if that makes Isengard better overall though.

Sorry to hear about your bike getting pinched.

What made you decide to go with Isengard after all?