The Karen Madsdatter in the 1787 census is your Karen Madsdatter. Here is my reasoning.
In the births in Sanderum of Hans (1775), opslag 117, Karen (1776), opslag 126, and Sara (142), opslag 142, the father is listed as Matz (Mads) Smed.
In the birth of Maren Olsdatter in 1803 to Ole Andersen and Karen Madsdatter one of the witnesses is Hans Madsen, opslag 236. In the 1801 Sanderum there is only one Hans Madsen old enough to be a witness and that is the son of Mads Nielsen.
In the birth of Mads Olsen on 28 Nov 1805, opslag 256, Mads Smed is a witness.
Mads Nielsen Smed, 61, died on 9 Oct 1810, opslag 61.
Together all this information proves to me that your Karen Madsdatter was the daughter of Mads Nielsen.