How exactly do ICAO's VOR-based minimum separation standards work?
Exactly. As long as the aircraft are diverging (i.e. flying away from the VOR), one of the aircraft is at least 15nm away from the VOR and both are at least 15 degrees (radials) apart, the separation is achieved.
You cannot use VOR separation with less than 15nm distance. The other option that can be used is "geographical separation", ref ICAO Doc 4444 5.4.1.2.1.1.
As they are still more than 15 nm away from the VOR, I would say yes. There would need to be however a certain restriction in place for the inbound aircraft to cross 15 DME (or preferably much before to have some margin) at FL50/5000ft or below, to ensure separation.
Yes. As far as one aircraft is 15nm or more away from the VOR, the separation is ensured. The photo in your question nicely depicts this, as it shows that only one (the furthest) aircraft has to be 15nm or more away from the VOR.
Yes, but you would have to ensure that they are vertically separated before the last inbound aircraft reaches 15nm from the VOR.