Missed the funny part
I think they intentionally played that angle. When Mallory is calling Archer's name repeatedly after shooting him, there is a pause, and then before they cut to the scene you describe, just when you think it's time to fade to black, you hear Archer yell "What?" in that irritated, childish,…

Shane
I'm guessing Shane's situation is more complex than did he know or not know Rick was dead. My take? He made a decision. Maybe he wasn't 100% sure. Maybe his feelings for Lori complicated it. That's something he's struggling to answer for himself right now.