used to be, all champs divided the xperience equally. Before that, everyone got the same experience.

Right now it looks like champs have a higher tendency to gain exp than soldiers (which I like btw), and sovereigns have a slightly higher tendency still).

In fact, forever I had been travelling with Sov+champ, but then recently I kept the champ in a newly conquered city. So I had a Sov + large army facing off against a very powerful (neutral, fire-beast army) foe. Well, my level 10 sov became level 12 after just that one battle

My understanding was that champions share/split xp with each other and units share/split xp with each other.  The idea being that you don't want to many champs in one army.

Champions share XP with each other, i.e having two in a party grants them only halve the XP. Regular units get halve of the XP the champion would get. XP increasing traits stack with each other. If they are from different sources (e.x., two champions with Trainer), they also stack. Bonus XP gets applied after the XP has been split. Using your example the XP calculation would look like this:

Sovereign: 5 base XP * 1.85 (1 +0.25(General) +0.25 (Knowledge) +0.25(Trainer II) +0.10(Trainer I)) = 9.25XP

The champion gets the same amount as the sovereign in this case

The units: 2.5 base XP * 1.60 (1 +.25(General) +0.25(Trainer II) +0.10(Trainer I)) = 4XP

I don't know how rounding works in the game, but, assuming it rounds down at the end of the calculation, your sovereign and the champion would get 9XP each, while the spiders and the demon get 4XP.

It is shown in game by hovering over the XP bar. As far as a table showing level points, don't think I've seen one.