I may be slightly off but I'll attempt to answer your questions. The union will argue that if the company allowed someone to work after being knowledgeable of an infraction, they must not have deemed it to be a cardinal infraction or they would have terminated immediately. If it is not a cardinal infraction, progressive discipline needs to take place. On the other hand, the company can say they did not have full information to make a determination at the time of being aware. Would potentially depend on the specific facts of the case.
As far as talking to someone without a Union Rep, it depends, again, on the specifics. If the company did not allow the driver to have requested representation than any information that resulted from that conversation can not be used against them for discipline. The company could still discipline if they had information independent of that conversation but past practice is such that they usually don't.
Weingarten rights, as laid out in case law and regulations, are largely misunderstood. A company can discipline an employee without union representation available but only if they solely impose discipline without any questions / interviews taking place. In the real UPS world, and probably other places, representation is made available regardless for discipline to avoid potential issues and the lack of representation has been accepted as an issue often by UPS Labor.
As for your specific case, we do not have all of the information available to make an educated comment. We don't know what the 'victims' stated to the company, the overall environment or a number of other factors. If it wasn't considered egregious, he could be in progressive discipline.