If I'm in the building around closing (12:00am) and they announce over the loud speaker that they are closed, is it a good idea to keep shopping anyways even though the polite, handsome, gentleman has informed me that the registers are closing?

Only if you enjoy the cavity search services provided by The Gloved One. Perhaps that is where the Smiley Face finally met his fate. Over in automotive. Right by the drum of lubricants. Violation on Bay One, The Inside Job.

No. Get out. At 5 minutes after, the employees are allowed to do physical harm and/or use weapons to clear the store

That is up to you a personal choice, i have been in that situation and i double timed it towards register but grabbed couple things on my way up there maybe 5 minutes extra to get up there, hope that helps.