Very interesting.

SQL does mark the physical records in a heap, AFAIK, with something to be sure they are unique. I have heard before that the optimizer likes clustered indexes, so they are recommended.

A table with one (or few rows), might take up more space with a clustered index, but the time it takes to discuss this isn't worth the space used. Just create a clustered index. It isn't enough space to worry about.

The nonclustered index for the PK means you create a heap, and then another index for the PK. A clustered PK is just the heap converted to clustered, no extra index.