Service tax cenvat sub lease

Renting of Immovable Property includes renting, letting, leasing, licensing, other similar arrangemnets  of immovable Property. Thus by defn, sub-letting is also covered. Thus the SErvice Tax paid by A to X can be treated as Input Service and the ST recd by B, on sublet can be treated as Output Service and setoff is possible.

For Ur 2nd question: yes, in  that case A will not have to pay Service TAx, but The return that is being filed, must show that CENVAT has been utilised for the payment of such OUput Sevice.

For ur 3rd Question, the relevance shifts to as to what you proposed to do with the other half. Kindly refer the text of "Renting of Immovable Property Service" for better understanding of the same.

1 more thing i wud like to add is that for ST to be applicable on sub-letting of property to B by A, it must be for "furtherance of  business or commerce", i.e. the property must be used by B for his business purposes AND A must have such business of letting out of such property for it to be treated as his Businee Income as well..