> how can a constituent know with absolute certainty that their vote was counted
The representative of your party plus independent observer said all votes at your polling station were counted. You know both those community members and know them to be generally honorable. Ergo your vote was counted.
> every voter in the system was legal
None of the observers at the polling station, or the station head claimed any illegal person voted.
> the final tally was authentic
The observers all signed as witnesses on the final tally.
This is not the "system. it is humans you know who are telling you what they saw. If you can't trust other humans at their word, democracy cannot fundamentally work.