Can you start maariv on Motzae Shabbat before Shabbat is over?
You can daven any time after Shkiah.
You need to remember to say Shema again if you said it before 25 minutes after Shkiah. (at least thats what my father told me years ago) You still can't do anything till the time that you wait for Shabbos to end. (72, 60, 45, etc.)