It seems that Aspinall is only destined for the interim title and he has to fight more fighters to be able to actually fight against the real heavyweight champion, who knows who will win between Jones and Miocic will definitely be the fighter that Aspinall challenges for the title.
Not quite, the purpose of the interim belt is to provide replacement if the undisputed champion is out due to an injury etc. But when he's back in action, there should be a unification bout between him and the interim belt holder.
So if Jones refuses to fight Tom, he will get stripped of the title in favour of Tom.
For some reason, the UFC still want to go ahead with Jones Vs Stipe, but, after that, Jones (or Stipe if he wins) will have to fight the interim belt holder or lose the title.
So Aspinall doesn't need to do anything to get the fight with Jones, unless he himself (or the UFC) doesn't want to be inactive for that long.
The UFC could force Tom to defend the interim belt, but that would be quite nonsense, as it would be like creating two parallel titles within the same weight-class.