Shopify EU VAT OSS Compliance

We build software that prepares EU One Stop Shop (Union scheme) VAT return figures for online stores selling goods to EU consumers. We need a qualified EU VAT specialist to answer three questions in writing, with sources:

1. Refunds of sales reported in an earlier quarter: does each member state of consumption require (a) a correction of the original quarter via Part 3 of a later return, or (b) a reduction in the current quarter's return? Rule for each of the 27 member states, as a table with a source for each.
2. Currency: when a EUR store's customer pays in another currency, use the store's recorded EUR amount or the customer-currency amount converted at the ECB rate on the last day of the quarter (Art. 366)? Any difference for non-EUR stores?
3. Rounding: VAT per member state and rate as the sum of VAT charged per order, or recalculated as total taxable amount times the rate? Any member state requirements?

Requirements:
- Qualified tax adviser (for example German Steuerberater, or chartered tax adviser with ADIT) with hands-on OSS experience. Please state your qualification and registration so we can verify it.
- Written answers plus the 27-country table for question 1, citing national law, tax authority guidance or the European Commission's OSS guide. No AI-generated answers.
- No competing VAT software providers, please.