Germans are shown as using poison gas in September 1914. The first use of poison gas in WW1 was actually in April 1915 at Ypres.
Marcel uncovers Caroline Dewitt's past as a prostitute by verifying her name on the official register of prostitutes, but how would he have known her maiden name or date of birth? Her brother-in-law Charles may or may not have known her real maiden name, but it is known in any case that Marcel did not speak to Charles about it until he called to share his discovery.