According to the Cambridge dictionary, uttermost is the formal version of utmost, which suggests that utmost was originally regarded as some kind of abbreviation.
You will find a definition of the origins of utter here.
Languages were not designed by a team of engineers: they have evolved over millennia. As with humans, evolution values diversity: it does not discard redundant or duplicated features, and only discards with extreme reluctance those that have a negative impact.
English developed over a large area in terms of the communications available at the time, and so duplication and inconsistencies were bound to develop. Once they exist, the evolutionary force that drives change in language is in no hurry to get rid of them.
While I am inclined to agree with fixer1234's comment, having done a bit of research on all the words you mention it seems that all of them lead back to origins with dis/des prefixes. It's just some of these words have taken a few detours in definition which don't make that connection as clear as some of the words you mentioned earlier.
For discover, the word originates from the Late latin discooperire, which breaks down into dis (opposite of) and cooperire (to cover up), as Trent mentions in his comment above.
For discuss, the word originates from the Latin discutere, which breaks down into dis (apart) and quatere (to shake). It seems that the definition shifted from smashing apart to scatter or disperse, then to investigate or examine and finally to debate.
For disguise, the word originates from the Old French desguiser, which breaks down into des (away, off) and guise (style, appearance - a word we still use today).
For display, the word originates from the Latin displicare, which breaks down into dis (apart) and plicare (to fold). This definition was used with regards to sails and flags (the idea of unfurling them/revealing them) and has gradually morphed to what it is today.
Best Answer
Affixes, like "un-", can be analysed in terms of how easily it is to form new word using them. Some are generally non-productive, for example "epi-" meaning "on" is a nonproductive prefix: You can't use "epidesk" and expect anyone to know what you are talking about.
On the other hand "anti-" is very productive. If someone is strongly opposed to (for example) fish, that person is anti-fish. It's a weird meaning, but everyone would know what you mean, in context.
On this scale, "un-" is somewhere in the middle. Many verbs adjectives can form a negative using "un-". If the verb describes a process, then the the un-verb means the reversal of that process, and can be used in new coinages. "Unparse" could be understood.
However before making a new word, you should consider the alternatives. Parsing is the process of breaking a sentence down and identifying the role played by each word. The opposite would be "constructing" a sentence. Perhaps this is a better word to use.
[The google chrome grammar and spell checker considers epidesk to be a misspelling, whereas it only suggests non-productive in place of nonproductive, indicating how productive the the prefix "non-" is. It allows antifish without comment.]