Today the mall closes at 6pm... so does that mean two underage people can’t enter the theater without an adult or can we?

Agree, yes. You should also always remember to park outside on the side of the mall where the movie theater is and enter there because the mall is closed once the movie is over and you could possibly have to walk all the way around outside at night.

Before 10 yes

I would guess that as long as you go before 10 PM, you should be alright.