Please read the documentation! The first style you were using had been deprecated for over 2 years as the download page clearly stated. Now you are asking questions about '[BO]' which are answered in the FAQ and remarks sections on the page you got
the new style from. Also, a simple search of this discussion group would have given you plenty of information on the subject...
Instead of the comment field, I would suggest using the tag field. It is what makes a citation unique. But you can of course stick to the comment field.
If you want your in-text citation the way you describe them, then you don't need an alphabetically ordered bibliography, you need a tag/comment ordered bibliography. That would mean that you don't need the extender and that you don't need the difficulties
In the style you downloaded, look for
and change all occurences in either
depending on where you store your tags.
Then, change all elements looking like this
<sortkey>some data here</sortkey>