Can a person paying tax under composition scheme make supplies of goods to SEZ

No. Supplies to SEZ from domestic tariff area will be treated as inter-State supply. A person paying tax under composition scheme cannot make inter-State outward supply of goods. Thus, for making supplies to an SEZ unit, a person needs to take registration as a regular taxpayer. The supplies to SEZ will be zero rated and the supplier will be entitled to make supplies without payment of tax or if he pays tax, he will be entitled to refund of tax so paid.

(FAQ 32: COMPOSITION LEVY)

In case the option to pay tax under composition levy is denied by the proper officer, can the person avail ITC on stock after denial

Yes. ITC can be availed by filing, a statement in FORM GST ITC-01 (containing
details of the stock of inputs and inputs contained in semi-finished or finished goods held in stock) by him on the date on which the option is denied as per order in FORM GST CMP- 07, within a period of thirty days from the order.

(FAQ 27: COMPOSITION LEVY)