You have to hand it to Nikolai for telling Sonya exactly how he felt and what the reality was. He isn’t leading her on, though he does have love for her, but it’s brotherly love.
The reason why Denisov is accepted and Dolokhov isn’t seems simple enough, but Natasha is astute in saying Dolokhov is calculating. Denisov is basically a Rostov: open, warm, impetuous so he fits right in.
Rostov’s = Weasley’s