Is it permissible to say "Wa Iyak" (and to you) in response to "Jazakallahu Khairan" (May Allah reward you with good)? And what is the similarity between saying "Wa Jazak" (and may He reward you) and "Wa Alaikum" (and upon you) in responses?

There is no impediment to responding with "wa iyyak" (and to you) to someone who has supplicated for your well-being or said "Jazakallahu khairan" (May Allah reward you with good), as its meaning is "wa anta jazakallahu khairan" (and may He reward you with good too). The same applies to "wa jazak" (and may He reward you). However, merely responding with "wa alaikum" to "Assalamu alaikum" is not permissible according to most scholars, because the response should be equivalent to or better than the initial greeting. Therefore, it is not appropriate to respond to the salam with "wa alaikum" only.