Hi Petr,
>  picking at least one intersection vertex results in doing fillet only on it
Yes. I think there would only ever be just one extension/intersection vertex, it would only appear for the case where you have picked 2 open curves to fillet, the ends closest to where you picked would get extended if the curves do not already intersect.
>  picking intersection vertex along with other internal corner(s) points
> results in doing fillet on each selected points
Yes.
> picking at least one internal corner with no intersection vertex selection
> results in filleting those selected internal corner points
Yes, the extension/intersection point will not get filleted in this case and the extension (you would see the curve extension on the screen during the vertex selection process) would be canceled.
> pushing "Done" button without picking anything results in filleting all internal corner
> points If I wanted to fillet the curves completely  both internal corners and intersection
> vertex points  I would have to select all of selectable points..
I was thinking if you pushed done without picking anything, it would always be equivalent to selecting all points. So that would mean if you had internal corners and an extension/intersection vertex, it would do all of those. If you only wanted to do internal corners and not the extension/intersection point, you would need to select all points except for that one. However, invert selection is available to use during this selection process so a shortcut would be to select the one point and do invert.
 Michael
