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#R, 118 114 bytes function(a)while(grepl("[^ ]",a)){F=F+1;cat(a,"\n");for(j in c("[^a-zA-Z]",letters[F],LETTERS[F]))a=gsub(j," ",a)}

Thanks to @Giuseppe for those 4 bytes off

Try it online!

Short explanation:

function(a)
    while(grepl("[^ ]",a)){ #As long as the string is not just spaces.
        F=F+1 #Increment letter counter (F is FALSE, hence 0 by default)
        cat(a,"\n") #Print string
        for(j in c("[^a-zA-Z]",letters[F],LETTERS[F])) #Get rid of non-letters, and the current letter in lower and upper case
             a=gsub(j," ",a)
    }