Composite dealer tax paid rate wrong and others

Sir,

1)One person selling of ice creams to consumers composite scheme registered.

Sales turnover jan-march,,19.    3 lacs

Tax rate of 5% but 1% tax paid wrong.

Different amount paid next quater April.march.19.adjustment.

2)One composite dealer 2018-2019 turnover 18 lacs.govt increase  40 lacs  limit gst registration. cancelation gst registration  gst reg form.16 applied. Dt.01.4.2019

Cancelation of registration after maintained account books compulsory or not.