Yeah, you're right. Would be used as a free Imbue, I'm assuming.

Also you can use something like:

    <GameModifier InternalName="Mana">
      <ModType>Resource</ModType>
      <Attribute>Mana</Attribute>
      <Value>20.0</Value>
      <Duration>0</Duration>
    </GameModifier>

for example to have spells output mana. Or add the <PerTurn>1</PerTurn> tag to give a certain amount per turn. Haven't tried this in tactical spells, though. Hope this helps!

That I don't know, but I'm guessing that this is left over from when mana was a local resource...

Modders can still use essence as a per-unit mana resource.

In the next release of expandedfactions there will be an elven unit called Bladedancer that can alternate between two stances, one favoring defense and one attack. This is how I accomplished that:

In the unittype, give the unit
<UnitStat_Essence>3</UnitStat_Essence>
<UnitStat_TacticalManaRegen>1</UnitStat_TacticalManaRegen>

then in the spelldefs use

<Manacost>3</Manacost>
(and make all modifiers duration 3)

The unit now uses internal mana to use these abilities. An ability costs 3 essence to use, which means the unit must see the start of another 3 turns before it can use an ability again.

Keep in mind to give the unit a very low intelligence (1), otherwise it will be able to cast spells from the spellbooks.

Yes, essence in this sense causes units to be able to cast strategic spells, but not tactical spells. They can still use manacost on unlocked abilities.