Is the first 12-15mm going to be plasterboard and plaster or is that sheet ply? If it's ply I'd be tempted to take the top 25mmish of it off and not cut into the beam, see how that sets them*, making good would be a good wood filler (right-hand top looks well off too but more like plaster there?!) . Standard disclaimer and I would be considered about their next problem if that was how they were left
Is the first 12-15mm going to be plasterboard and plaster or is that sheet ply? If it's ply I'd be tempted to take the top 25mmish of it off and not cut into the beam, see how that sets them*, making good would be a good wood filler (right-hand top looks well off too but more like plaster there?!) . Standard disclaimer and I would be considered about their next problem if that was how they were left