March of the Monarchs vs Negate Tribute Summon
Yes, Player B can activate Solemn Judgement.

Note the activation timing of Solemn Judgement (emphasis mine):


  When a monster(s) would be Summoned,


I.e., the monster's summon has not been completed.

It is therefore not yet a Tribute Summoned monster, and receives no protection from March of the Monarchs.