Read snYpir's "How to use Objectives and init.sqs" in the Editors Depot. If you want hidden objectives you need to learn how to use init.sqs. Fortunately it's easy.
Yes it is true that you can synch triggers and units, and you get new options in the Activation box.
setMarkerType "empty" to make the marker invisible. (Do this in init.sqs, see you're using it for something else already.) When you want it to appear, setmarkertype to whatever you want.