I have just started into Constructs and so far haven't seen anything that uses mana. Is there something in Constructs that uses mana or am I able to tear up my mana buildings and put something else there?
You need Mana for producing the working units in the Central island and for various upgrades, such as Trader, Main Hall and the new AWs. Maybe for other things too. So don't be fooled by the fact Mana and Seeds are not needed for research.