> Mobbing geht ansonsten tatsaechlich immer von mehreren Personen aus
This is mistaken in my opinion. Frank, I'm afraid you've fallen into the classic 'false friend' trap here, by assuming that [D.] Mobbing has something to do with E. 'mobbing'. The two words have very little in common other than their spelling.
In claiming that this always involves several people, you're undoubtedly thinking about the basic meaning of the English word 'mobbing', which derives from 'mob' and has to do with lots of people--indeed, it's the very meaning of the word 'mob'.
However, the German word 'Mobbing' which translates as 'Workplace bullying' in my opinion has nothing to do with English 'mob' and typically involves one person (the bully) victimizing one person or sometimes more than one. Nothing in the definition precludes the possibility of one or more bullies joining up in a cabal or conspiracy, but this is likely a small minority of cases.
For further definitions from those concerned with this topic full time, click 'Oft Gefragt' on http://www.mobbing-web.de/
or many other sites having to do with these topics.
Your explanation of 'bad mouthing' is correct, and might be part of a pattern or a constellation of activities of a workplace bully, but as an isolated event is too mild to constitute 'Mobbing' all by itself.