I've never played any of the Paradox games so I don't know how easy they are to mod.
Which Paradox game is easy to mod? I know GalCiv2 is easy to mod, it's just like Elemental (and a Stardock game). Mount and Blade is another game I've played... and it is NOT easy to mod at all. Anyways, I don't want to debate this, lets get to your problem.
Modding Elemental requires a few rules:
1. Don't try to modify any Core files
2. Use either the Mods directory or the Units directory for implementing mods
3. Start a new and fresh game, savegames don't work
4. Some detective work
Regarding Mana Regen, I believe the game sets it's regen at 1 by default in one of the other files (CoreDef.xml maybe?) but the reason your not seeing any change to it is because it is disabled in the CoreUnitStats.XML file:
<!--<UnitStatType InternalName="UnitStat_ManaRegen">
<DisplayName>Mana Regeneration</DisplayName>
<DisplayNameShort>MGEN</DisplayNameShort>
<Description>Unit regenerates this amount of mana per turn.</Description>
<Icon>ManaRegen_Stat_Icon.png</Icon>
<Hidden>0</Hidden>
<AffectPerLevelUpPoint>1.0</AffectPerLevelUpPoint>
</UnitStatType>-->
A "<!--" means that it is disabled.
My advice for you, if you want to get it to work, is to make a Mod.XML file in the "Units" folder. Place the above code into it (with ALL the necessary coding), removing the "<!-- -->", then make a copy of the CoreSovereigns.XML, place it into the "Units" folder, and try making your sovereign modifications there.