Do Python and Java lead students to construct different mental models of memory?
I am not aware that cognitive modeling of memory structures in early programming education has been directly studied, so anything that I say here is entirely speculative.  However, I suspect that the models themselves would somewhat depend on teacher presentation, and would otherwise be relatively stable.

Take negative indexing as an example.  A reasonable initial cognitive model for a list would be some sort of cycle.  If it is later explained as an array with indexing performed with a modular operation, then we have moved our conception to some sort of line.  When we even later explain about memory references and data size, then we can finish the picture as an array (line) of fixed size memory references, each pointing to a different object in the heap.

There are only so many simple physical shapes that we can use to represent these ideas.  Which actual conceptualization a student takes will depend on their maturity in the subject matter and on how the data structure is presented by the instructor.  I don't see any reason to believe that the initial choice of language would have any substantive, lasting impact on these cognitive models as the student matures in their understanding.