Of course those things affected the vote -- although GOOD did far worse than many anti-DA posters predicted (the biggest prediction was the DA would lose its majority in the WC because of De Lille departing) -- but the leader is ultimately responsible for his party's performance and the DA's internal review did suggest that Mmusi stand down, as well as other two leaders.
They still managed to grow in other areas during municipal elections, culminating in enough votes to form a coalition government with smaller parties in Tshwane, Jozi and NMB. How did it go wrong from there?
The wrecking ball that is internal politics hogged headlines, followed by the de Lille PR disaster. Zille followed with ill-discipline on twitter, refusing to listen to the party leadership. This also kind of exposed Maimane and painted his presence in the leadership as a mere facade. During that time, ANC's propaganda machine was hard at work, doing enough to sway some of the new voters away from the DA. It's going to take a while (if ever) to get those back.