Thank you for the insightful suggestion. In Sitefinity 3.7 there is such functionality to view the individual Content PlaceHolder IDs that are set in my Master Page while editing template. Please consider Sitefinity 4.x is a complete rewrite of the source and some of the thing which were available in Sitefinity 3.7 are not currently implemented in 4.x . Currently to preserve the content of your pages after template change please refer to this
Regarding your first suggestion Sitefinity uses the widgets which take their content from modules and you can just drag a widget corresponding to the content you want to show to the desired template and all content is preserved because it is accessed from the module. So we have presentation and data layer.
Regarding your second suggestion I can`t really understand what for is this shared content between content sections needed please clarify your example.
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