Is it a coincidence that the sensory systems for smell, vision, taste, and hearing are all near each other in the head?
No it's not a coincidence that eyes, nose, mouth, and ears are all near each other in the head. As you note the layout is inherited from an ancestor, that's your answer.
 

A study of vertebrate head sensory systems shows that the


  The olfactory, vestibular auditory, and lateral line sensory organs displayed a strong link with the urochordate-vertebrate ancestor. 


So this paper highlights that these structures go back earlier than the animals in your list. The development of vertebrates is very complicated so major departures from this developmental program which are successful are going to be rare events. This is reflected by blind cave dwelling animals that have vestigial non-functional eyes. The lack of selection pressure means the eyes don't work but they are still there because building an eye is part of a deep developmental programme. The image below shows (in orange) the key drivers of differentiation for various cell types. It makes it obvious that the program that produces olfaction also produces vision (otic) so modifications to the developmental program will have wide-ranging effects.  



The theories that you've listed are interesting and there is work going into developing new senses. Udo Wachter wore a belt that vibrated to north 24/7, "north" became part of his dreams, he always knew where home was, and he mourned the loss of the the sense. So proximity to the brain is not a critical factor for the senses.