Whether a registered person who has paid IGST on a transaction considered by him to be an inter-state supply, but which is subsequently held to be an intra-state supply is required to pay interest

No. Such registered person shall not be required to pay any interest on the amount of CGST and SGST or CGST and UTGST payable, as the case may be.

(ICAI FAQ PUBLICATIONS 06-09-2017 Tax wrongfully collected and paid to Central Government or State Government : FAQ NO. 38)

What happens if a registered person has paid CGST and SGST or, as the case may be, CGST and UTGST on a transaction considered by him to be an intra-state supply but which is subsequently held to be an inter-state supply

Refund of CGST and SGST or, as the case may be, CGST and UTGST shall be granted in such manner and subject to such conditions as may be prescribed.

(ICAI FAQ PUBLICATIONS 06-09-2017 Tax wrongfully collected and paid to Central Government or State Government : FAQ NO. 36)

In case the person does not deposit tax collected in contravention of Section 76, what is the course of action available to the proper officer

The proper officer shall issue notice requiring him to show cause as to why the amount so collected as tax should not be paid by him to the Government and why a penalty equivalent to the amount specified in the notice should not be imposed on him under the provisions of the Act. The proper officer shall adjudicate the matter and issue order within one year from the date of issue of the show cause notice.

(ICAI FAQ PUBLICATIONS 06-09-2017 Tax collected but not paid to Government: FAQ NO. 32)