Breto:

Mustelvulpo:

EldanarLambetur:Li kuris trans la ŝtonojn, stumblis, malstumblis mallonge, sed fine tute falis. ~ He ran across the stones, stumbled, briefly found his footing (un-stumbled), but finally completely fell over.

malstumblis mallonge - I get it but I have to pause to think about it. Something like "momente stabiliĝis" comes across more clearly. It seems like every aspect of not functioning is covered by nefunkcii and malfunkcii is rarely if ever called for. Similar to the idea that something can be neblua but malblua is pretty much impossible to define.

Something is either blue or it is not, something either functions or it does not. There really is no opposite.

I dunno about that. Thanks to negative images, we see the opposites of colors all the time. Arguably malblua is simply oranĝa, and likewise malruĝa would be verda and malflava would be purpura. Of course, just as we don't say malnorda for suda, there's no real reason to say malblua for oranĝa.

For funkcii, though, I agree with you. I can understand funkcii "to function". I can also understand nefunkcii "to not function", or misfunkcii "to malfunction", but malfunkcii "to unfunction" is not something I can really wrap my head around. As for nefunkcii meaning "to malfunction", I'm not aware of the predominant usage, but to me "malfunction" has always meant that something is still being done, just incorrectly. Misfunkcii seems a better translation of this concept than nefunkcii, which seems as though it could describe a perfectly functional machine that is simply not being used at the moment.