Your points are well taken and yes, there are times when official definitions are ignored, which can be very problematic. For example, more recently, "racism" has been redefined as "including power," basically meaning that only white people can be racist. As a social psychologists that drives me nuts because racism is a term used in the social sciences and carefully defined for the purposes of research. But problematic isn't necessarily fallacious.
BUT why is it really wrong to use as you say “a definition of a term as evidence that term cannot have another meaning, expanded meaning, or even conflicting meaning" from a dictionary at least as part of an argument?
Because absence of evidence is not necessarily evidence of absence, in other words, if one dictionary leaves out a usage of the term, it does not mean that excluded usage isn't valid. The fallacy is in the assumption that your chosen dictionary is the ultimate authority on the word being defined. In terms of science, this would be like one person pointing to one study as a scientific fact. Of course, we need to look at other studies, better yet, meta-analyses which can give us a much more accurate picture. Dictionary use needs to be the same. Generally, this is not a problem but sometimes minor differences can have drastic effects on an argument.
So why is there a need for singling out dictionaries?
Because like scientific papers, if the definition is vital to the argument being made, it's a good idea to consult multiple sources... including encyclopedias and scientific papers where the term might have been operationalized.
If there is this concept of appeal to definition that singles out dictionaries, shouldn't there also be a mirror fallacy that warns against irrational avoidance of dictionary definition?
Perhaps there should be. Like I mentioned, I have been seeing this more and more. I am not sure how common it was in the past, but in our "post-truth" world people seem to be fine with making things up. But is this fallacious, or just a manipulation technique by the ones ignoring the dictionary definition? Those who buy into the made up definitions, are they being unreasonable or are they simply ignorant (not knowing the actual definition) or too trusting of the source? Maybe a fallacy should be created.