Technically true, of course. There's exit polling in the picture as well, though. If one machine or group of them produces an odd exit result, the machines and the Internet trails before (or after) the vote can be examined for hacking by geek investigators. That will tend to implicate individuals and organizations that would get in trouble.
I think the rule for election-stealing hasn't changed that much since the paper-ballot or lever days: Steal relatively small numbers of votes not outside the margin of exit-polling error, but preferably on a large number of machines.
Indeed. Small nibbles do not get noticed. A Big bite does. Embezzling tends to happen within the margin of error. Remember the old John Cash song about the guy who takes Caddy parts out of the factory in his lunch box and eventually has enough to build his asymetrical Car?