Birthright:

Does starting academy educated also cost 6 years for a shorter lived race or does it scale with the lifespan?

It costs 6 years for all races. It is a real question of value for Gnolls, for sure. There is of course always the choice to buy Long Life as a signature blood ability.
As a descendant of Mange, the Undying - that is not unreasonable.

With the extra Regent XP do Lts start at a higher level than 1? The "within 5 levels" for taking them on adventures is why I ask.

If they attended the academy with you, and are the appropriate age, yes.

Is the an alternative to the Dain Academy? The bonuses seems too good to pass up, makes think the other shoe is going to drop.

You are paranoid! Go get the highest quality education of all time!

The alternative is not attending. It's worth considering why no other alternatives have survived, and how much longer this one will.

The alternative is indeed not attending any academy. It is the only Academy in the game at start, though of course any number of domains could decide to build new ones in game.

Sad to see the Academy in the Isles apparently died.

There were a few, I got rid of most buildings, so that people had choice(s), and for competition on buildings to be possible.