Firstly – no the EC can not overrule a decision taken at an AGM. Simple.
But if the common seal has now been applied to a contract then this significantly complicates the situation. This needs to be addressed by others in this forum who know way more than me.
But in an aside – I think you have good reason to lobby for support to replace the current strata manager, and put in someone who is more ethical and responsible.
Not sure how big your complex is, but you could find that with a decent strata manager, you may not even need a building manager. I would suggest that a building manager should not be allowed to write work orders, that all work orders are approved by EC delegates (or sub committee) and then written by the strata manager. I hope that a legal eye went over the building managers contract for sticky issues as there is no standard industry contract, as building managers are completly unregulated. But you probably already know this.
Good luck
Alley cat