"The situation is far worse in the case of Whitehead’s ontology, where it is said that every entity in the universe shares a perfectly definite “prehension” (relation) to every other entity in the universe and that each entity is but a bundle of the way in which it prehends other entities. Whitehead says that he wants to think the conditions under which novelty are possible, but it is difficult to see how there could every be any novelty in his ontology for the very simple and basic reason that there can never be any new encounters for entities. Why can’t there be any new encounters between entities? There can be no new encounters between entities because entities are already related to all other entities that exist in the universe. Where an entity is already related to all other entities that exist, there can be no question of a new encounter."
I have a problem with this literalizing of Whitehead’s notion of ‘prehension’ in this sentence: “The situation is far worse in the case of Whitehead’s ontology, where it is said that every entity in the universe shares a perfectly definite “prehension” (relation) to every other entity in the universe and that each entity is but a bundle of the way in which it prehends other entities.” Obviously this leaves out the fullness of Whitehead’s use of the four notions that underpin his ontology: actual entities, prehension, nexus, and the ontological principle. Of which it is ‘nexus’ (togetherness), not prehension that involves relations in the sense you attribute to prehensions. As summed up in this statement:“
Actual entities involve each other by reason of their prehensions of each other. There are thus real individual facts of the togetherness of actual entities, which are real, individual, and particular, in the same sense in which actual entities and the prehensions are real, individual, and particular. Any such particular fact of togetherness among actual entities is called a ‘nexus’ (plural form is written ‘nexūs’). The ultimate facts of immediate actual experience are actual entities, prehensions, and nexūs. All else is, for our experience, derivative abstraction(p. 20).”
I think you have to take into account the complex of those two notions, prehension and nexus, without which one or the other as an explanatory principle means nothing. It is nexus that is descriptive of the relations (togetherness) among objects, while prehension is a mode of internal analysis. So in this sense Whitehead would almost agree with your internal/external distinction, he would just define it as prehension/nexus with the caveat that he is describing an abstraction not an ontological fact.
Being an Idealist and a monist he affirmed the Platonic distinction of form and fact, as he states the case: “…philosophy is explanatory of abstraction, and not of concreteness. It is by reason of their instinctive grasp of this ultimate truth that, in spite of much association with arbitrary fancifulness and atavistic mysticism, types of Platonic philosophy retain their abiding appeal; they seek the forms in the facts. Each fact is more than its forms, and each form ‘participates’ throughout the world of facts. The definiteness of fact is due to its forms; but the individual fact is a creature, and creativity is the ultimate behind all forms, inexplicable by forms, and conditioned by its creatures(p. 20).” The engine of creation is creativity, the power of creation, energia, force. That every fact is a creature, and that every creature is potentially a creative agent in the world is the one unique aspect of Whitehead's philosophical freedom. Without this engine of creativity at the heart of all creatures there would never be any potential for change in the world. It is this creativity that aligns freedom and the event within all actual occasions.
To continue… As for how change is possible. Whitehead defines his notion of the ‘event’: “Thus the actual world is built up of actual occasions; and by the ontological principle whatever things there are in any sense of ‘existence,’ are derived by abstraction from actual occasions. I shall use the term ‘event’ in the more general sense of a nexus of actual occasions, inter-related in some determinate fashion in one extensive quantum. An actual occasion is the limiting type of an event with only one member. … The fundamental meaning of the notion of ‘change’ is ‘the difference between actual occasions comprised in some determinate event.’(73)”. He explicates this use of event: “An event is a nexus of actual occasions inter-related in some determinate fashion in some extensive quantum: it is either a nexus in its formal completeness, or it is an objectified nexus. One actual occasion is a limiting type of event. The most general sense of the meaning of change is ‘the differences between actual occasions in one event.’ For example, a molecule is a historic route of actual occasions; and such a route is an ‘event.’ Now the motion of the molecule is nothing else than the differences between the successive occasions of its life-history in respect to the extensive quanta from which they arise; and the changes in the molecule are the consequential differences in the actual occasions(80-81).”
As you can see his ideas on change are based on the notion of the event, which includes difference in its abstract argument for how change comes about through the temporal differentiation of actual occasions in the event or fact.
Whitehead, Alfred North (2010-05-11). Process and Reality (Gifford Lectures Delivered in the University of Edinburgh During the Session 1927-28)