We've been playing that the 5 gold color bank is always available as long as you have cards to trade in for money, regardless of whether or not other players have already used the color bank space.
Correct. The 5 gold bank spot is the one spot on the board that is always available, no matter what has happened in the round so far. The game *must* have a spot like this, since every player must play every single card they have. Without that spot, you could reach a point in the game where it was impossible to continue, as a player could have colours in their hand that were unplayable otherwise.
We've also played that you can dump any number of cards at once for 5 X (# of cards) in gold.
No, I don't think you've got this one right. It's one card per turn:
"If a player places any buyer card face down on this building on his turn, he receives 5 Asari in return.
Everything is singular, one card, 5 bucks.