You're right that it will be much better if you can set these properties on per shape level, but this also may have a lot of side effects for example if you select multiple shapes and some of them can or can not be modified and also other issues. But still I added work item for this scenario so that we can provide better API. You can track and vote for it here.
As a different workaround you can handle the SelectionChanged and if you encounter that a shape shouldn't be modifiable - you can set the diagram property and after that you can return it back to the previous value.
All the best,
the Telerik team
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