There are different errors in both posts.
In the first one - H2 cannot exist in the presence of strong oxidizer - then you should expect not H2 but product of reduction of HNO3 - NO or NO2 depending on concentration of HNO3.
Moreover, this concerns both posts, Hg2(NO3)2 itself spontaneously decomposes to Hg and Hg(NO3)2, and Hg can react with diluted HNO3 to form Hg2(NO3)2. Since the decomposition is not extremely fast, in the case of diluted acid you can expect both nitrates (of course then the reaction is difficult to balancing).
Concentrated acid is needed for conversion of Hg2Cl2 or other Hg(I) fluorides to soluble Hg(II) nitrates.
All mercury salts with acids that contain oxygen are predominantly ionic than you can balance redox by a half reaction method using Hg22+ and NO3-