Which, as I already said, has more to do with complicated sites having unforeseen vulnerabilities.
The technology isn't much more complicated than the pin code on an iPhone, or a password. The trick is simply making the pin long enough. Like, the program to hack past all possible pins for a four digit password is seconds, iirc. For 17 digits, it takes years. By initiating the three strikes and lockout protocol, it's basically impossible to hack into. On the other end, it is closed to other connections, just tabulation and recording votes with registered voters in the district.
Conceptually it shouldn't be particularly challenging.
As we have seen, no. You often need a few thousand in key areas to swing a vote. We had a problem with mass challenging without the voters knowledge way back in the Reagan Era. Paper voters didn't even know their ballots were being tossed by the thousands. The same happened with Bush.
Eta: and hundreds of millions? Really, dude? Neither candidate had even one hundred million votes TOTAL, and the regular voting kept it close to 50/50 anyway. Election throwing is a few thousand in the right place, not hundreds of millions.