In related news, Saudi Arabia hacked Bezos' phone: https://news.ycombinator.com/item?id=22122779

Could a similar hack be done on a larger scale, and used to alter votes before they are sent, without changing the electronic signature?

Lets assume no until proven otherwise. And if proven otherwise, lets assume it was an isolated incident, and it won't happen again after a "commitment to do better".

Lets also assume Mr. Tusk's firm can be trusted - after all, why not put a private middle-man between voters and elections? As Mr. Tusk correctly points out, the only plausible explanation to resistance is wanting to suppress voters.