A key difference between C79 and PVA is that C79 shows amounts of import VAT that has already been paid over to HMRC, and you can just reclaim on your VAT return. PVA statements show import VAT amounts that have not yet been paid to HMRC, so entries are required to both pay it to and reclaim it from HMRC via the VAT return.
I don’t know how any specific software packages would need them to be entered – but, from the VAT return perspective, it’s very similar to the reverse charge on services. The VAT figures need to be included in both box 1 and 4.
The adjustment should be done in the return for the period covered by the statement. I don’t know what months each of your VAT return quarters cover, but if you have statements going back to August, some will now be late. The normal rules in section 4 of HMRC’s notice 700/45 apply – regarding whether the correction can be done on the next return, or needs to be notified on form VAT652. Generally errors under £10k can be corrected on the next return, with higher limits (up to £50k) applying to larger businesses.
Do you know for certain that August 2021 was definitely the first? The system will only let you download the last six months statements, older ones have to be requested (see: https://www.gov.uk/guidance/get-your-postponed-import-vat-statement#if-you-need-a-statement-more-than-6-months-old ).