By métro, from the Mercure, it’s more 10-12 min and not 30. Only 2 stops (from Vaugirard to Porte de Versailles). Walking will be indeed around 25 min.
Although I understand the convenience of staying very close to the convention place, do not expect miraculous Diamond benefits at either Mercure or Novotel.
If I were you, I would call the Novotel and ask them to contact you in case of availability. Unfortunately, I have seen a very strong tendency now for Accor hotels to not monitor anymore the generic email address mentioned on their website.