Mystery of the blue train

So, I finished this book. I liked it!

But im confused about some things, it felt like the end of the book is rushed, like yeah, Poirot explains everything, but I didnt understand some things.

Katherine said that Ruth made a decision, but what was it?

What happened to Armand? So, he just wanted the rubies, and thats really it?

Did Katherine and Derek get together? Im pretty confused with her romance stuff, she said that Knighton is handsome and she even blushed around him, was that an act?

What was the thing with Mirrelle?Did she lie bout seeing Derek? Did she see the body and ran away?

Was it ever explained how Derek knew bout the rubies?

And a silly thought, did Lenox ever confess her love to Poirot? Or was it just admiration?

I was genuinely confused reading many things, I also mixed up some names... Also since I was reading the russian version, I couldnt even think about the cigars belogning to Knighton, bruh

Anyways, it was a okay book. I honestly wanted more, like some kinda twist that reveales Katherine or her father being the killers, though I actually didnt have any explanation for that besides it being a shock, yknow?

Anyways, Im getting closer to the murder in orient express, and maybe thats why Im underestimating these books, cause I heard AC wrote them when she was having a bad time and stuff.