Have to say this resonates with me.
Your senior dog needs a safe escape, and to be able to take herself away from the puppy. Their breeds and needs are going to be different.
Labs are very social, generally tending towards extrovert, dogs. And as a young puppy, she will have no manners, know no boundaries. Your older dog may well help with that, but to be successful, she needs to be allowed to set the pace.
That said, look at some pheromone calming products. I have used Adaptil collars and plugins before with some success ( both dogs would need a collar so check if they are suitable for young pups.
Make sure each dog gets one-to-one time. And get your puppy enrolled to start classes as soon as she can mix with other
I hear you on the buyers remorse thing. My first lab, a male, was hard work at times, but oh so worth it.