Difference between はもとより and はもちろん and their respective usage

I would like to know if there is some difference between　はもとより and はもちろん　in the sense of “let alone” , “not only … but also”.

On a side note, I would like also to know their respective usage (oral/written, normal/formal, modern/archaïc)

今の子供たちは学校はもとより塾へも通わせられている。
Children nowadays are not only forced
to go to school but also scram school.

わたしが生まれた村は、電車はもとより、バスも通っていない。
There are no buses in the village
where I was born, let alone trains.

わたしの学校は服装に厳しい。制服の着方はもちろん、ヘアスタイルやスカートの長さに至るまで注意される。
My school is
very strict about clothes. They even warn us about our hairstyle and
the length of our skirts, let alone how we wear our uniforms.