[Plugin: Co-Authors Plus] wrong manage_columns authors link for custom post types

This is already fixed in an upcoming version.

Great. Not sure, how did you do it, so just one quick idea – you should use add_query_arg like

echo '<a href="'.add_query_arg( array('author' => $author->ID) ).'">'.$author->name.'</a>';

This way it would work even with any other parameters, so e.g. i could do search and then filter found posts by author.

P.S.: I know that you probably know this ??

I think the default behavior of core is to simply drop existing query args. For instance, if you were already filtering to one author, you wouldn’t want to filter to a second author.

if you would do this as i wrote it, it would actualy REPLACE the author as it is the same parameter as in current url, so your example would work.
IMO if a user first filter by category (or do search, or other filter) and then click an author, he wants to filter by both. If not, he can simply clear the first filter.
another option would be to add both, e.g. something like [Tomas Kapler][+], and clicking the plus would add it to query and clicking just a name would replace it. It could be shown this way only when there is some param (other then posttype) in current query, so it would not confuse ordinary users.