Overstay Japan Visa By 1 Day

Hi! I'll be arriving Japan this Wednesday (Nov 20) at 11pm. Then I booked a plane going back on Wednesday (Nov 27) with my family.

However, my boyfriend and I decided to extend our stay so we booked another flight to go back home at a much later date on December 6 (Friday) 7pm.

Now my stay will be for 17 days (Nov 20 to Dec 6) but I read that they don't count the arrival date, which makes it a 16 day stay.

I only recently noticed my visa was valid for 15-day stay while my boyfriend's is valid for 30-day stay.

Is it possible to still push through and go through immigration on our flight back?

Essentially, you would be overstaying your visa, which is illegal. Ask for an extension before you leave, or re-book your flight to match your approved stay.

The period of stay granted at the time of landing permission which means once you got the landing approval. Extension you need to have a valid reason since only 7 countries may extend waver visa.
Other option is to change your flight schedule.