Seriously, I do not see how any faction besides Pariden can amass it.
Um, no. Anyone with a Death archmage has as much mana as they'll need for most purposes by casting Sacrifice. Magnar can get lots of mana early on by casting Consume, and almost unlimited amounts of mana by casting Cull the Weak on Skeleton Horde (although I consider the latter an exploit and think Cull the Weak should banned for summoned units (or only for Trained units, as is the case with Death Lash)).
Pariden can get a decent amount of mana, as you note. For other factions without access to Sacrifice, yes mana is scarce, but if you cast Meditation, build and upgrade shards and build enough Conclaves, I generally find I have enough for all except the most expensive spells.