What is the ruling on burial in a "nowamah" where no soil is placed, and does it take the ruling of a regular grave regarding the permissibility of sitting on it?

The default rule for burial is that each deceased person should have a separate grave, and more than one deceased person should not be buried in a single grave unless there is a necessity. Burial in vaults (underground chambers) is prohibited because it violates the sanctity of the deceased. However, if excavation is impossible, there is no objection to burying in them.

As for sitting on the grave for reasons other than answering the call of nature, it is mildly reprehensible (makrooh tanzeehan) according to most scholars, based on the hadith: "Do not sit on graves," and the hadith: "For one of you to sit on a burning coal... is better for him than to sit on a grave." The grave that is prohibited to sit on is one that is directly above the deceased or very close to them. The ruling on sitting on vaults does not differ from the ruling on conventional graves; thus, it is reprehensible to sit on the part of the vault that is directly above the deceased.