Election day is open to oversight from all political parties. All parties send their people (identified with badges, previously registered with the voting authority) and they have free access to all voting locations.

At the end of the day, the bag of paper votes is transported with their oversight (multiple parties from opposing sides) to a public place where all of them will count the papers, together, in a public session, with cameras.

It's actually what already happens when the e-voting machines fail and your backup also fails. You cast votes with paper and the votes are counted with this exact same process, with oversight from all parties.

The paper vote records (anomymous, of course) become the source of truth. There are very well established methods (Risk-limiting Audits - https://en.wikipedia.org/wiki/Risk-limiting_audit) that can randomly sample a small fraction of the ballots to guarantee with high probability that the electronic results are correct. Without paper records, the source of truth lies with the code that was run to receive and register the votes, and that is almost impossible to verify and fully trust.