Here in Australia, ballots are (initially) counted at the place they were cast. Every ballot issued has a corresponding (but unlinkable) person on the roll (electors are crossed off the roll prior to a ballot being issued). The count of names crossed corresponds to the number of ballots issued which will correspond to the number of ballots counted at the end of the day. Virtually impossible to "insert" a ballot as then your final count would exceed ballots issued.

As the count is completed at the polling place, of which there are multiple per electorate, the total number of votes is substantially lower than 43,000 even, and so the % accuracy is much higher. No publicised figure (that I know of) for how many "missing" ballots there can be before questions are raised, but I suspect its in the single digit range. Ballots are generally counted a couple of times but, even more if there's any ballots missing.

> If, at any time in counting process, an individual could successfully insert a single valid but fabricated ballot into the process

This varies by jurisdiction obviously, but where I'm from the procedure for counting must be done in an area which the public can access and it begins with a single person taking ballots out of the box one by one and giving it to a chain of 2-3 other people.

This way you can count how many ballots were taken out of the box and check with the totals at the end

And obviously the box is always in the presence of observers from various stakeholders

> less than 43,000 votes

Unless you can predict where these tiny margins will manifest with perfect accuracy you'd need to add a lot more fake votes, or at least have thousands of conspirators ready to add them at a moment's notice, that's ridiculously hard to organize discreetly

I’d argue that gerrymandering is a far bigger issue than ballot stuffing. Its sole purpose is to ensure that elections go in favor of the party who draws the map. If that ain’t rigging an election, I don’t know what is.