Fix the Meeesesessessesseesseessedessed upp teeexexextext
CJam, 207

{_,1&gt;{:E1&lt;_0{:I2$,+E=:C+:R1&gt;C'9&gt;)*+:P,E,&lt;{EP#{L0}{PRI)1}?}{PE#L{R8cP,E,-*+}?0}?}g}&amp;}:U;LqS+'a+{_'[,_el^A,s+&amp;,V={+}{s\V!:V{L{:BU_aL?B,,1&gt;Bf{_2$&lt;U_{_W=8&gt;S8c+*+\@&gt;j+}{?;}?}+{,}$0=}j}|\}?}%s'\8cN++'\"\bn"f+er-2&lt;


Try it online

Explanation:

Almost forgot to write this :p

The problem is solved in several steps:


the text is separated into runs of alphanumeric characters (let's call them words) and runs of non-alphanumeric characters (non-words)
non-words are printed as they are, and words are fixed
fixing a word is done recursively (with memoization): split the word into 2 chunks in every possible way (including empty 2nd chunk), try to unmess the first chunk (see below), fix the 2nd chunk and join the results with a space-backspace if needed; take the shortest sub-solution
unmessing a chunk means finding a minimal string of alphanumeric characters possibly followed by some backspaces (but with no backspaces in the middle) that when messed up, result in the given chunk; it is done with a simple greedy algorithm, going from left to right, building the unmessed string and its messed version in parallel, and determining the next needed character at each step; some chunks can not be unmessed