Deferring Admission to reapply for some PhD programs next year

This question is related to one of my previous questions:

Changing University in First year of Phd

I just completed my senior year. I have been accepted to a grad school (say University X).

I want to reapply for some PhD programs next year (fall). But, at the same time, I don't want to lose the only PhD seat I've got.

So, I was thinking about deferring the admission to University X in order to apply to few other universities for next fall.

I have 2 questions regarding this:

1. 
   

   Should I mention about my deferred admission to Uni X in the Phd applications for next fall ? If I don't, Would it be treated as academic cheating ?

   
   


2. 
   

   How much deferring should be enough ? (Next Spring, Next Summer or Next Fall)