To 1: Sex Scene can the the right way to begin a story. In some Sci-Fi Universe like Battletech and even Star Ship Trooper, Sex is a casual Thing between comrade. With the always looming thread on the horizon (plague) and focus on military and war - life is short and cheap. So the characters in this universe enjoy while they can.
To 2: When i look on the first day you are entirely correct. After the first dinner on day one between the two, the day basicly ends with just a deal. This can be easily fixed with some extra narration. However your asssements. However its a crucial mistake by both parties not just Calbex. On day two, Calbex provide moral support. After that he is cut off by the MCs tamper tantrum. Thats my only issus with the story: Dispite all the MCs dissapponitment by the bombshell Calbex dropped at him, you dont throw a fit at your Knight! And when Calbex called after him: "MC, wait!" the MC didnt listen. If Fen would have been the Knight, the MC would have been punished for disobyeing a direct order. The MC himself admits he acts like a child.
Fixing:
Day 1 evening can be easily /lazily fixed if Calbex just give the MC some Files or workout routine to learn. Can be done in just a few sentences...
Day2 + 3: The Outburst by both MC and Calbex seems to be a vital part of the story and their behaviour. A little bit more dialog my help, and not the "get angry and go to bed with bad mood - refuse to eloborate" - approach.