Hello Nick.
I'm mighty impressed with the research you are putting into this! I built 'Mars' according to what was visible in the Burt book and this area remained a mystery. I presume you have Admiralty plans to work to?
So far as I know, there is no surviving builder's model of a Majestic. If anyone knows to the contrary, please say!!!
However, there are models of various 'Duncans', including 'Russel' which was on display at the IWM at the time. Below is a snap looking down into the engine room area. There are hatches there, perhaps not quite the same layout that you are working to. Are those the same ones? Details in the 'Duncans' were generally very similar to the Majestics. I know the builders models don't always tell the truth, but in my experience they are a lot more reliable than re-drawn sketches in Janes or even, Burt.
Here's 'Russel':