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Citation needed...

I don't think laws are ever that prescriptive about what you have to do. As far as I know, the only requirement is when the tax inspector arrives, you need to be able to produce a complete and accurate list of all invoices you have ever sent to customers.

at least for Germany: digital invoices shall be processed in a way that manipulation can be ruled out (GoBD). Hence companies use special scanner and document management systems which document the Revisionssicherheit.

And that is the new, more relaxed situation. At first, digital invoices were required to be signed with a qualified signature, i.e. spend a few hundred quid a year on certificates by a few select CAs (only the usual german "suspects" could qualify due to intentionally onerous requirements).

They aren't editing an invoice, they're using a template to create a new invoice.

Even before issuance? No such requirement in Finland at least, so clearly not an EU level requirement.

The Finnish VAT Act does, however, require that companies sending and receiving invoices ensure that the mandatory invoice data is not modified after issuance (209 g §).

Source?