First of all, really like Sahi's idea. The rivalry between him and Lune is going to be epic and fun too. I can see them working together if they have common interests, but otherwise, they will seriously be at odds I think. The MC will havta find a way to make them work together at times and that sounds like a cool thing for the plot too (as long as there isn't tons of it all the time cause that would not be good for the kingdom lol).
About the fight question ... I don't know. My best prediction is that they're pretty evenly matched. Sahi is a great tactician, but I can totally see Lune using tricks and talking to mess with Sahi and making this a draw. And the best part would be that they would draw and hate it anyway lol. Just sounds fun and slightly awful to watch though too lol.
Can't do anything with the armor here. I do always like a good warrior though so there's that.
What's Sahi's personality like (a bit more detail, you did give some already). What position did he originally have with the guards? I know he's the general now, but if he has something like ... well a cross-specialty (fighting of course for this) maybe have him do that for a bit and his lt. can take over for him for a bit. Like if he's good with training and the army needs new recruits trained quickly, he could step in to help out with that for example in a crisis situation maybe. Just thinking from a tactical point of view here and if I were the king/queen here and there was a crisis with needing more recruits for the guards or with needing to train new people, if my general was good at that fore example, I would use everyone I could for training people.