Your question is not clear to me. You might be asking why the Gemara says that the first 8 Amos of the 20 Amos that are checked, are checked diagonally across the 4 x 6 rectangle. You thought that this means we are checking the central cavity of the cave (Me'arah) off of which the Kuchin (recesses) branch. Your question is that the dead are not buried in the cave, but in the Kuchin.

If that is your question, you are correct; there is no need to check under the Me'arah. The eight Amos are the diagonal of the 4 x 6 area of the Kuchin (recesses), which are each four Amos deep (to accomodate a person's height and then some) and extend 6 Amos along the wall of the Me'arah.

M. Kornfeld