As hard as it is for you, you should wait until 16 weeks of age for any kitten. I know that many vets and some books, etc say 8 weeks is OK. But many say that 8 weeks is the absolute minimum.
When my cat had her kittens, I asked my vet and she said 8 weeks minimum. I found a good home for 2 of the babies quickly and told them they could have them after 8 weeks. Leaving 2 kittens behind with mommy cat and me.
I did not find a home for the 3rd kitten until he was 16 weeks old. During that 4 weeks I could see both babies learning social skills from each other and from their mom. They behaved better, they began greeting me more, they learned when their play was too rough and they were less fearful of new people. THese are the kinds of social skills a human cannot really teach a cat. Only another feline can really teach them. They matured a lot in that 4 weeks.
Be patient. You can probably work something out with the breeder where she will let you visit. And if she will not let you visit while waiting, then you should think twice about how reputable a breeder she is.
Overall, you will end up with a much better adjusted cat if you wait. Babies need to stay with mom and their litter mates as long as they can.
And I vote yes on a second kitten. Kittens need a playmate. This is my first time with 2 cats also. 2 cats is no more work for you and twice the fun. I will always have 2 cats from now on. I think everyone needs a companion of their own species to play with.