Cess-refund to sez

No 

Your query related  to section 54(3) & proviso under subsection (4) r/w rule 89(1) & 89(2)(l)

Yes you are correct  the 'doctrine of unjust  enrichment "is not applicable upto the amount 2 lac but if it is exceeding,  then ca  certificate  is required. 

Note : from 1/2/2019 u/s 54(8)(a) the word  "Zero rated" has been substituted  by "Export" and Exports, thus with this amendment,  in case of refund claim arising out of supply  to Sez is also being brought  under principal of UNJUST ENRICHMENT