Sir, Yes you are right As I seen you haven't shown your sales during 2017-18 at all. Some where around you need to show the sales details otherwise your customers wont get credit of it. It should have done for the moth of march 19. As per my limited knowledge you can show the details in GSTR 1 for the moth of June/July 19 (not in 3B), because they extended due date for GSTR 9 but not to amend the sales details.
This gonna be considered as Sales/liability declared in subsequent year of filing annual return. In case of any query raised by the officer he will be convinced with the explanation on practical difficulties.
And Cash lying in your E-cash ledger cant be considered as taxes paid for 2017-18, unless you set off against the liability declared in DRC 3 for such period.
This is purely based on my opinion .