Hi,
I am unclear on your question.
When you place a block-level content control in a cell, the w:sdt (and w:sdtContent) surround the w:tc element, as your image shows.
Are you saying that when you get the root element, you see within the root element a table with only one row? This seems pretty surprising to me, and I suspect that I am misunderstanding your question.
You may be interested in using the Open XML Package Editor PowerTool.
See screen-cast #4 in the following series:
Introduction to Open XML
Best, Eric