Unaffiliated Hero Mana

So here's two semi-related questions:

Mana comes from a Sovereign with access to shards, correct?  So how do heroes that have yet to sign up to follow a particular Sovereign have any mana?  I'm curious, since it's counter to the lore. 

How is each unaffiliated hero's mana calculated?  I fought a level 3 guy at his camp and he cast burning hands 7 times in a row.  Do they have unlimited mana or is it just a fairly large amount?   Is it safe to assume those independent heroes will always have mana for another spell if that's the optimal action?