Editors choose layout
You're asking a chicken and egg problem. Nodes just hold data and no information about the theme (usually). You're describing mixing the two in the Body field of a node, in essence hard-coding the look (layout) of your "node" in the database.

Drupal in many respects fights this as the page content is 1 item and drupal7 cares nothing about whats around it. In D8 their is a UX initiative to make a nicer page layout creation system.

In D7 you may be interested in context_layouts found in Context. See this youtube video on its usage.

I've done something similar (but not Grid aware) using Spaces, spaces_node and Dashboard (spaces dashboard) in Drupal6 per node (I also used node_block heavily). Every node/page could have their own unique layout using Spaces and its submodule of Dashboard; you can set any content to any block/region of the page as needed using a drag-and-drop overlay. Actually, the site I'm mentioning is UCSB SageCenter. Unless your logged in and editing a Space/Dashboard you dont really see anything in the source. I added a node field to pre-assign a known space for node pages on node creation. This site was hard.

Here's a video demo on spaces_node -- its not supported anymore really.