I think egosyntonic comes reasonably close:
Egosyntonic is a psychological term referring to behaviors, values, feelings that are in harmony with or acceptable to the needs and goals of the ego, or consistent with one's ideal self-image.
Many personality disorders are considered to be egosyntonic and are, therefore, difficult to treat. Anorexia nervosa, a difficult-to-treat Axis I disorder, is also considered egosyntonic because many of its sufferers deny that they have a problem.
Its counterpart is egodystonic.
I think John Lawler and others make a good point in that "antonyms" are vague, and I suspect that, despite the descriptivist intent, the question arises from a semantic issue.
From Wiktionary, an antonym is "a word which has the opposite meaning of another, although not necessarily in all its senses." Thus fast is an antonym of slow, but fast is also an antonym of eat. However, most of us wouldn't think about comparing speed with consumption. Useful can be interpreted as "having non-zero utility," which means the opposite of useless. However, useful can also mean "having a positive degree of utility" which is not the opposite of useless. So they are fine antonyms, but not opposite in all meanings. A more appropriate opposite for the comparative version of useful would be harmful or detrimental.
For the more descriptive questions, specifically regarding the "-ful" and "-less" suffixes, I suspect that use of these words depend on how these suffixes are commonly interpreted. "Doubtless" and "useless," for example, imply devoid of doubt and devoid of use. "Thoughtless" and "tasteless," for example, imply lacking thought and lacking taste. The latter pair would be more common in comparative relative to non-comparative use since one can be naturally seen as more or less lacking. The former pair is less commonly seen since it is less logical and descriptively less common (though not unthinkable) to be seen as more or less devoid (of course, cf. emptiest). In general the commonality of use seems to me in line with whether or not it is logical -- so I don't see them as necessarily in conflict.
However, one exception comes to my mind (not saying that there aren't others). When raukh mentioned "impossible" (p = 0), my first thought of an antonym was "certain" (p = 1). As someone more accustomed to speaking with statisticians, for me, it sounds awkward when someone says something is more or less certain. However, I recognize that both descriptively and formally, certain is a comparative adjective. Indeed, it seems that the use of certain as a comparative is more common than the use of uncertain as a comparative, although that appears to be in relative decline.
Additionally -- this is perhaps silly of me to think it needs stating -- choice of which words to use also depend upon the emphasis of the sentence, even for paired words. Whether someting is "more impossible" or "less possible" may, for some, have different connotations. Curiously, those words seem to be converging in frequency of use.
Best Answer
Word pairs like love and hate, right and wrong are gradable antonyms:
The words you're looking for are midway along the spectrum between gradable antonyms, so medionym would be a good neologism. However, there's another way of looking at this.
Love (in the sense of passion or ambition) is also the opposite of indifference – just in a different direction from the love–hate spectrum. Likewise for right and ambiguous. The words you're talking about are still antonyms, of a special sort that is opposite to both ends of a spectrum. A good name for this would be orthogonal antonym, to reflect that it is “perpendicular” to gradable antonyms.