For the markers, set the marker you want to hide in the editor to "emtpy" and then use the command setmarkertype to change it, see below.
As for the objective, in your init.sqs file type:
"obj_X" objstatus "Hidden"
and when it should show up use:
"obj_X" objstatus "Active"
Where X is the number of the objective.
markerName setMarkerType markerType
Operand types:
markerName: String
markerType: String
Type of returned value:
Nothing
Description:
Set marker type. Type may be any of: "Flag", "Flag1", "Dot", "Destroy", "Start", "End", "Warning", "Join", "Pickup", "Unknown", "Marker", "Arrow", "Empty".
Example:
"MarkerOne" setMarkerType "Arrow"
:beat: *Gets Shot* :beat: