I’ll try my best to help you 
As you’re on the flat rate VAT scheme, I think this post may be able to help you resolve this.
As for the other points, for the figures generated by the VAT return, you would need to have a look at the backing report, which can be accessed at the bottom of the return itself using the ‘Download Calculations’ button. I’d highly recommend that you check this prior to submission anyway, just to be sure. This will outline anything that’s been used to calculate the figures for the return, and would help you notice any issues or errors.
