Whitening toothpaste remove the discoloring from tea in my dentine exposed bottom incisor tooth?

Not likely. Exposed dentin and cementum (root) does not respond to whitening. The enamel does. To get a better cosmetic result, you will need to get a tooth colored restoration (filling).
Not likely. Dentin is naturally a yellowish color and the addition of tea stain is not an easy combo to get white with just tooth paste of any kind. A good cleaning and in office or take home bleaching from a dentist is much more concentrated and likely to provide some success.
Yes, but... It may lighten the stain but whitening toothpastes a famous for causing tooth sensitivity, particularly when there are exposed roots. A better solution may be to consult with your dentist?