I'm also not sure about whether the 5 before L 21 rule applies to prestige classes.
As for whether or not Battlerage will kick off the terrifying rage aura, I have no idea. That said, if I have a moment, I can make an NPC test character and see if it works. Outside of that you can ping Orth or maybe HB on Discord and see if they can look at the script to see if it will work.
I just tested this with my battlerager and it does trigger the aura. Thanks!