Unmake is indeed a word. It is a verb meaning to reverse, or undo the making of. Synonyms include to ruin or destroy.
The use of these words, unmake and sensible, together is uncommon. A better phrase to use would be: "to confuse," "to ignore," or to "be unreasonable, 'depending on the usage.
Answer:
of a person or their behaviour) unconventional and slightly strange.
Yes, it seems to be an alternative, in some situations acceptable, but eccentric and not neutral spelling of this word. It accents the fact that "ea" here are two separate vowels rather than a diphthong: that is the function of the trema (the two dots).
In practice it's not used much, because it's not really necessary: it does not distinguish it from any other word, and trema is not easy to find on English keyboards: so there is no need.
