Nobody's stopping you using "whelmed". You can just start using it whenever the opportunity arises.

I use it occasionally, though normally not seriously, along with words where you have an "un" or "dis', but no positive equivalent, e.g. "gusting" as a positive "disgusting"

Whelm and overwhelm are synonyms.

The way I learned it, is that people have a tendency to emphasize, so when became overwhelmed. You see the same thing with 'good', 'great', and 'epic'meaning the same thing within certain contexts.