What is the ruling on performing Sa'i in the basement (qabw), and is there any issue with this Sa'i, and what is the scholars' opinion on performing Sa'i below the masa'a (Sa'i path)?

The Sa'i (ritual circumambulation) in the basement is valid and there is no harm in it, because the air follows the ground level. So, if a basement were opened along the entire length of the Sa'i path, performing the Sa'i in it would be sufficient.