A vital plot line, obviously, is that Doyle wants to kill the con men who fleeced his runner of the numbers money. He has Luther killed and turns his best men (and women) onto Johnny Hooker, almost killing him, too. What about the third conman, Kid Erie? He is an essential part of the con, as much a part of it as Luther and Hooker. During the setup - just before they fleece him - the numbers runner watches Kid Erie running away. He looks at him, Hooker and Luther in turn. Even if he couldn't identify him he would still be able to inform Doyle that there were three rather than two con men involved. Even so, Kid Erie comes and goes as he pleases. Doyle doesn't have anyone looking for him; he doesn't even mention him in conversation, and in fact consistently refers to two - not three - con men. He makes it clear that he would have to kill his best friend if he even found out about the con, yet he lets one of the central participants go scot free. It doesn't make any sense at all.
Doyle Lonnegan's limp was a result of Robert Shaw injuring his ankle. Rather than working around it, Shaw incorporated the limp into his performance.