Metro Line 4 from Montparnasse to Strasbourg-Saint-Denis, then Line 3 toward République. Walk the last 5 minutes to Rue des Gravilliers.
Use Metro toward Gare du Nord for the Brussels train day if you keep that plan. Give yourselves extra time with bags.
Evening fallback route is Metro 11 to Châtelet, then RER C to Champ de Mars-Tour Eiffel. Cab is still the cleaner move at night.