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That's because you are using the part of SageManifolds that is implemented in SageMath 7.2. This part is very limited and does not include smooth manifolds yet. Indeed SageManifolds is being gradually incorporated in SageMath (see https://trac.sagemath.org/ticket/18528). Hopefully smooth manifolds should appear in SageMath 7.3.
Meanwhile, the solution is to install the full SageManifolds atop SageMath 7.2, following these instructions.
The answer to your PS(II) is yes: see here.
