Would this work out how it seems? Like would I pay 30 and gain 2 resources points? or is it imited to once per turn or something.

no, the card doesnt allow you to pay more than 15 END.
if it allowed you to pay 30 END to gain 2 resource points it would use differant wording and specifically say that you could.

dont assume a card allows you to do more than its says you can

no, if you had two seprate Conjuration.

well, yes thats fine, there is no restriction about how may you can play, if there was, it would be clearly printed on the card