Would I let my Empire fall? No. But neither would I use Black Rose. I know my history, the first time the Allies used lethal gas weapons against Germany in WWI was after they capture stockpiles of mustard shells from the Germans and returned the favor. To use Black Rose is to have Black Rose used upon your Empire. It's just a matter of fact that powerful weapons end up captured by the enemy over time and reverse engineered or simply commandeered. Black Rose is an escalation from a war of survival to a war of extermination, to use is it is to doom the Empire.
The Empire invades and conquers far distant countries and the Garleans think they are the ones being oppressed? That's too stupid for words. Gas weapons are not weapons of last resort, they escalate the conflict which is the last thing you want to happen when the war is going badly for you. If the war is going badly, you want to deescalate the conflict so you can get an armistice to save the country before it's invaded, not strengthen the cause of the enemy.