Depends on which option you've opted. If you're paying 12% tax on forward charge basis then you can take ITC as appearing in 2A (subject to fulfillment of conditions) or else you're not allowed to take ITC if you have opted for 5% option..
Yes you're liable to pay 5% GST (CGST 2.5 +SGST 2.5 or IGST 5 as the case may be..) through e-cash ledger... you can't use credit ledger to pay RCM tax