Nikaah & Marriage Issues

Bridegroom mumbling some words in Arabic at the time of nikaah

Q: Was this nikah valid?

The Imam told the bridegroom clearly to many witnesses in the masjid that ‘do you accept so and so’ upon which the bridegroom mumbled a few words in Arabic (as the bridegroom was a Talib Ilm so probably wanted to say the Arabic phrase). The mumbling could be heard by the audience.

Does it matter if the audience didn’t understand what was being said by the bridegroom?

Girl under 18 getting married

Q: I am 17 years old. My mother has passed away last year and I am living with my mamo and nano. I am totally burdened to them. My sister got married last year. They want me to also get married. I am biologically 19. My age was change in childhood. Can I get my nikaah since I've heard that girls under 18 cannot get married?