Conjunctions

Karan writes "He is very rich and he is not happy", thinking "and" can join any two ideas. His sister says a different joining word fits better here. Who is right?

Manav writes "He was tired so he worked late", trying to say that working late made him tired. His friend says this gives the opposite meaning. Who is right?

Zoya writes "Although he was tired, but he continued working", thinking both words together will make the contrast stronger. Her brother says one word needs to be removed. Who is right?

Ayush writes "Neither Rahul or Sameer came to the party", thinking "or" can pair with "either/neither" anywhere. His teacher says "neither" has one fixed partner word. Who is right?