1.Yes ITC is available to the merchant exporter on goods procured at a concessional rate by him, circular no:37/11/2018 dated :15.03.2018, even the person making a supply at a concessional rate to the merchant exporter can take credit on inputs procured by him
2.In the case of high sea sales , they are neither supply of goods nor service as per schedule 3, as there is no specification for reversal and not included in the value of exempt supplies under rule 42 and 43 , explanation to 17(3) of the CGST act, no need to reverse credit, I. e. credit can be taken