Soldiers with horn-helmets, special abilities, flame-throwers...

Are any of these new things in Kingdoms historically accurate, or has the game gone off into fantasy units like it did in RTW?

As far as I know yes.
The horned helmets are accurate
http://unitienne.altervista.org/imma...c_knight_1.jpg
And so is the flame thrower.

Horned helmets: the only myth is that every viking had one. They have been used.

Special abilites: well, we don't know exactly what they are, so we can't really judge them. But anyway, great generals have tended to do great things on the battlefield, so they are appropriate in my opinion.

Greek flamethrower: this is the greek fire, mainly a naval weapon. They have been used in towers, but I am not sure if they have been used in field battles. Even if they didn't, it isn't a very far stretch in realism. One of those things that "could have happened".

Well they had greek fire from catapults to light up siege towers during the crusades, so a flamethrower isn't too far off depending on your interpretation

Suprising as it sounds they are.

Vikings didn't have horns on their helmets, but the Teutonic orders did - check google for the Prankhe helm.

A period picture of the flamethrower:
https://img54.imageshack.us/img54/1240/greekfireyy1.jpg

Special abilities are most likely representations of famous character's actual abilities in an unrealistic way so as to work within the confines of the game.

I'm guessing special abilities along the lines of "irresistible charge", "rally all routers" and "impenetrable defense"...

Some of those have been confirmed in the other topic already.
The confusing enemies strikes me as odd though - how is a general going to make enemies fight each other during battle just like that?

Aura of Dread or something along those lines could fit though. Like an ability linked to the rally routers ability so that when you activate it, it not only attempts to get your toops to rally but also reduces morale of enemy units nearby due to the fearsome presence (high dread) of the commanding general.

Horned Helmets were used during the Medieval era to intimidate. The vikings used them for only special occasions, but they were used in battle, particularly in the crusades.

There were some flamethrower like machines in the Medieval period, some even dating back to ancient times.

Special abilities in them selves are not really real but have a basis. One could build up a reputation for being brutal, and inspire terror in the enemy, or have a very strong down cut.