Preparation of Mercurous nitrite
1.Why is it difficult to prepare it, and how to prepare it?


Prafulla Chandra Ray prepared mercurous nitrite, $\ce{Hg2(NO2)2}$ by reacting metallic mercury with cold, dilute nitrous and nitric acid. According to his paper(1):


  Mercurous nitrite is the product of the combined action of nitrous
  and nitric acids on mercury : 
  $$\ce{2Hg + NO2.H + HO.NO2 -&gt; Hg2(NO2)2 + H2O}$$
   Some of the nitrite is decomposed by the nitric acid, the quantity
  of nitrous acid thus rapidly growing, until mercurous nitrite and nitrate
  are accumulated in molecular proportions. After some, the nitrous acid remains constant in the solution. It has become the catalytic agent between the nitric acid and the mercury,which now interact in the following manner:
  $$\ce{4Hg + 4HNO3 -&gt; Hg2(NO2)2 + Hg2(NO3)2 + 2HO}$$


It is difficult to prepare the compound because many mercury-nitrite species is generated in the solution like  $\ce{Hg(NO2)2}$, $\ce{Hg2(NO3)2.2H2O}$, $\ce{Hg2(NO3)2.OH.Hg(NO3)2}$ etc. These salts exist in the solution and it is difficult to separate the main salt i.e mercurous nitrite from these salts and isolate in pure solid salts as all salts have similar properties and they tend to impure the solid mercurous nitrite.


  2.Can it be obtained by simply mixing $\ce{Hg2Cl2}$ and $\ce{AgNO2}$, where $\ce{AgCl}$ will be precipitated?


Reaction between $\ce{HgCl2}$ and $\ce{AgNO2}$ to yield mercuric nitrite $\ce{Hg(NO2)2}$ has been reported (2) but reaction between $\ce{Hg2Cl2}$ and $\ce{AgNO2}$ to yield mercurous nitrite $\ce{Hg2(NO2)2}$ is yet to be reported.

References 


http://pubs.rsc.org/en/content/articlepdf/1905/ct/ct9058700171
http://chemiday.com/en/reaction/3-1-0-5880
http://www.insa.nic.in/writereaddata/UpLoadedFiles/IJHS/Vol49_4_3_AChakravorty.pdf
http://nopr.niscair.res.in/bitstream/123456789/11011/1/IJCA%2050A(2)%20137-140.pdf
https://www.researchgate.net/publication/289285311_On_mercurous_nitrite_and_a_basic_mercurous_nitrate_derivative