Since in open-source one can only setup notes to be accessible by all/none/owner and in case you don't want to share all the notes/attachments with every single employee, you must set it to "owner" i guess.

The problem appers when you don't see the notes that were created by customers (via portal username). You can't see those, because you can see only own notes. So noone but person who has access to all notes/attachments can view the customers notes. The self-service portal is therefore useless for open-source which lack team management.

Is there any cheap solution to allow usernmae to which the case is assigned, to see the notes/attachments created by portal (customers) ?

Also how do one see that some cases were modified by the customers (portal), I haven't noticed any indicator for that...