Let me get straight to your questions. So, to achieve your requirements:
1) Zoom operation - you can subscribe to the diagram's PreviewZoom
event and check if the Ctrl key is down, if not, you can cancel the zoom operation by setting the event arguments' Handled property to True.
2) Scroll operation - you can use the diagram's MouseWheel
event and change the diagram's Y position with the e.Delta.
For your convenience I implemented this approach in the attached project. Please take a look at it and let us know if it works for you
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