Okay, this is interesting: I'm actually really surprised to say so. At first glance this is your basic "fake a marriage to fool the (grand)parents", but the MC isn't just going along with an unreasonable situation and the situation actually doesn't seem that unreasonable or terrible. From the setup I expected at least his family to be kinda mean to him, but they aren't. Those were some nice parting words from the brothers too. His internal monologue called him useless, but it seems to go a little deeper than that. That feels true for everything here. In theory we have the basic puzzle pieces and a rudimentary solve: she wanted a vegetable Husband to get out of starting a succession dispute that might lead to factions, internal strife, and even a civil war. Okay, but the MC isn't really a vegetable? She could've picked some lazy and fat and dumb greedy noble and been done with it. She could've picked someone charmed and enthralled by her (the Manga sets her up as very beautiful and someone many people admire). Instead she went for seemingly a random person whom she even paid for and the Manga makes it out that there's some reason why (even if that just turns out to be the "secret childhood friends"). I don't feel lost but there's interesting mysteries to solve to keep my reading and the characters feel human enough (not too perfect or too infuriating) that I'm not against reading more.