Branthwaite is a much better fit for most teams, including us, than Antonio Silva. There would also not be problems with managing his and Martínez's playtime, as both of them can take up that LB role that Zinchenko does in Arsenal, or the likes of Aké and Gvardiol have at City. There's also always injuries, suspensions and fixture congestion. Branthwaite would be so much more, than just a backup for Martínez.
I don't see us selling Shaw, who's better than Hernandez anyways, he just needs to be managed better due to his injury issues, and with 3 good options in that position (Shaw, Malacia, Martínez, potentially our new LCB will also be able to play there), will we break the bank for a new left back? Amass will also almost certainly be given a real chance in our first team, in a season or two.
Two right footed centre backs is different to two left footed centre backs, mostly due to the passing angles, which is important to ten Hag, as we know, as well as every top manager out there. There's a reason you almost never see two left footed centre backs partnering each other, unless it's a makeshift solution. I've only seen Luis Enrique set up like that. Every top team ideally has a left footed CB and a right footed one next to each other.