Note, to make a sentence more parallel simply means separating the details to allow easy understanding and remembrance by the readers of what was said.
So it could be written as;
To become a professional musician involves taking:
thousands of hours of practice and
finally working hard to make connections in the music industry.
It would be the girls’ dogs since there are multiple owners of the dogs. You keep the girls plural and use the apostrophe at the end to make it girls’ since it is both plural and possessive.
