Thanks for the kind words, glad you are enjoying it!
Adding 1 defense to your mine from Mithril Forge can only be used once per game, yes.
You only need 1 ore forge to be able to pay 3 ore to add 3 to your attack. In other words, like you said, one ore forge can use any amount of ore to modify your attack.
I hope this helps, thanks for checking the game out!
-James