Doc 4444
I would agree with your interpretation of a). In your example the departure would be vertically separated underneath the arrival. Then you could use 5.4.2.3.4.2 to establish a pass and allow climb/descent. (Although with jets this is all going to happen pretty quickly and may not be practical.)
It is a case of one separation leads to another....
Not everything in DOC 4444 is straight forward. Even the Figure 5-38 introduces some items that do not match the text.
Most countries publish their own interpretations and these tend to be more practical. (Although not always). Take 5.4.1.2.1.2 a). (I have seen this interpreted in about 4 different ways in local ATC regulations.)
VOR: both aircraft are established on radials diverging by at least 15 degrees and at least one aircraft is at a distance of 28 km (15 NM) or more from the facility (see Figure 5-2);
Because of the word "diverging" some countries insist that it can only be applied to outbound aircraft. (This is supported by the Figure 5-2 which shows outbounds). Others apply this to IN/OUT and/or IN/IN. The distances and number of degrees used varies greatly as well.