Can a word be used incorrectly and still be 'useful'?
Interesting question, thanks!
A word used in a deviant way would only have meaning if those who listen or read understand it. For example, I sometimes get confused in casual conversation and come out with a spoonerism -- a mixed up word. Usually, though, my wife understands what I mean anyway, by interpolating from a shared context. A new word, or a new meaning of a word, might gradually come to be accepted usage more widely. Slang words, in particular, tend to get picked up rapidly in this way.
Let us say that the 'correct' meaning and usage of a word is determined by the dictionaries. But a modern dictionary is itself only a reflection of broad usage by speakers and writers. We have to go back quite a way in time to find a dictionary that sets out to adjudicate meanings, rather than simply record or describe them. So, the 'official' meaning of word comes about because of common usage. If enough people use a slang word, it ends up in the dictionary. Dictionaries tend to be a conservative force in language, a kind of brake if you will, but they do not bring fix the language in place. I'm rather fond of old dictionaries, for just this reason: as a guide to how language appeared for this or that historical writer.
This is true of 'subjective' and 'objective' -- in fact, the dictionary that happens to be on my desk right now (Collins) comes very close to listing the incorrect meanings you identify as possible meanings of the words, without qualification (I mean, without describing the meaning as 'casual' or 'informal').
The danger here is that subjective and objective are words loaded with philosophical meaning. If a word like 'funk' -- which originally referred to the smell of enclosed spaces, especially of cigarette smoke; the cubbyholes along the sides of WW1 trenches where the rank and file of soldiers ate, slept and smoked were 'funk holes' -- comes to describe a genre of music and even an attitude, then there is a net gain in the resources of language. However, if subjective and objective become used to simply mean characterised as opinion and fact, then something has been lost. For instance, it becomes literally impossible to employ these words to argue about epistemological realism, without further comment. That is why, in philosophical writing, it is important to define your terms up front -- to do in miniature what a prescriptive dictionary would do. I just finished teaching Heidegger's Basic Problems of Phenomenology this semester, and one of the things he talks about there is the way that this loss of meaning of subjective and objective makes it difficult to articulate the intentional structure of human consciousness (which would have to be described as both subjective and objective).