::I do not know how the word "proposition" is used in philosophy or linguistics, or in philosophical logic.  I leave it to you to fix that part.  I do know how the word is used in mathematics.  Mathematicians could do without the word entirely, using "statement" for the non-boldface sense and "Theorem" for the boldface sense.  However, in the discussion of the way a word is used in philosophy, it is (for me at least) often helpful to compare and contrast the way the word is used in other contexts. 

 ::For example, I gather from your additions to the article, in philosophy it is considered important to distinguish whether the proposition "is" the idea, or "is" the string of symbols that represent the idea.  Mathematicians are aware of this distinction, but there are few mathematicians active in this area (none that I know of).  Most working mathematicians I know tend to get impatient with such distinctions, and to consider all possible ramifications of that distinction to have been investigated to exhaustion in the early 19th century.  Else, instead of the long example I gave at the end of the previous talk section, we would have to say "The number the idea of which is expressed by the string "2" added to the number the idea of which is expressed by the string "2" is equal to the number the idea of which is expressed by the string "4"."  From a pragmatic point of view, once you know the distinction is there, you can go ahead and work with ordinary language (albeit a highly rarefied ordinary language) and ignore the distinction between the idea and the symbol, not because it isn't an important distinction, but because everything that can be said about it (in mathematics) has already been said.