Henchmans real difference is they do not divide combat xp. The code basically tells the game they are for all intesive purposes champions.

One could set the Henchmen to be unable to learn any of the elemental spells (I think is an appropriate nerf to their abilities, as they are supposed to be lesser champions)

You are not confused about anything. Henchmen are supposed to do exactly what you suggest and why Altar in general is a very good faction to play.

I have no idea what the argument against removing the champion XP split is with henchman in the game. This is a super powered faction in that they can produce hero-plus troops for twenty fame a pop, but that these troops are immune to the split and champions aren't makes champions look like towel-boys. Why overshadow such a prominent game mechanism like that? 

There is no argument about ruining game balance by taking out the split while henchman are in the game. You can do things with henchman that are ridiculous sheerly because of the level they can start at, the number of them you can produce, and how quickly you can produce them. Heroes come slow, are pre-leveled in a way you may not like, and because of the split are just a pain to manage in comparison. So they tend get parked in cities for a nominal unrest bonus. The most underwhelming part of the game is when you get a surprise champion from a quest, which is supposed to be a wow moment, and instead you just have to spend 20 or so turns building up an army for them, if you even bother. Why is that the desired design? What does Stardock have against their legendary heroes that they make them look like chumps to henchman?

I wrote a big thread arguing against the split and the best answer anyone could come up with is play Altar. Why can't other factions have fun heroes too?

I too was puzzled about why henchmen were so much better than heroes when I played Altar (that was back in FE but the same basics applied).

Henchmen are a cool idea but they need to be balanced appropriately so that in a part of a couple of heroes and several henchmen the heroes are still, well, legendary!

IIRC, another disadvantage for henchmen is that their spell books are limited.  Not really a big deal, all things considered.  I think if they could die (which is in a Parrotmath mod, I believe) they would be fine.

Henchmen are created with the Rush ability.

Their initial spell traits are limited to just 3.   For example, you don't get Life Apprentice but Life Adept (Aid, Heal, Growth)

They have another advantage that I will not reveal.  If I do, you all will want to nerf it.

The disadvantage is that it costs fame and gildar to create them; and usually crystal and metal.  You also need to do significant Research before they are available.   Same with the Decalon to give them spellbooks.

That is bad prioritization, if the purpose is to preserve the uniqueness of the Altar race. That is making an integral part of the game a pain in the butt so it can be fun with 1 out of 10 the races. It is also completely unnecessary. The fact that you can play Altar and not even try very hard to have 3 times or more as many henchman/champions as any other race will keep them notable for their hero stacks. The bonuses they can get from turning unlimited henchman in to gildar/research towns unto themselves trump any other faction's racial abilities alone, but the combat strategies you can use with super-leveled henchman armies blow any semblance of game balance out of the water.

I don't know what you do with your heroes, so I can't comment on the fun you have with them.

Not referring to that.