I was wondering, Is your filler cushion covered with the same fabric as your two berth mattresses, your V-berth mattresses and your salon couch cushions?
If not, then imo the previous owner likely had something made, to fill the space between the two berths. I have never heard of a "filler cushion" as a Trojan option for the space between the aft berths, although it is indeed an interesting thought.
I went aboard my former TriFly (she's still berthed at our marina) and took accurate measurements of our enlargement of the port berth.
We gained seven inches in width by encroaching into the passageway. (In the process, we moved the aft entry steps a couple of inches toward the starboard bunk). Other than that, there was absolutely no impact on how the passageway itself functions. You won't even notice it.
By the way, we re-used the teak combing that extends along passageway the length of the berth . At either end though, we spliced in a "longer" piece of replacement combing, in order to create the added width. I used three quarter inch plywood for the base. I extended the width of the base a few extra inches over the top of the engine compartment to provide some additional stability.
Removing the hollow wall on the outside of the berth will give you around 14 extra inches. But because I wanted to provide a finished looking appearance and also wanted to keep the engine room heat out, we re-boxed the newly created space with new teak panelling. When we were finished, we'd gained ten more inches of width. Therefore the total increase in width was seventeen inches.
Now that I've actually removed that hollow wall, I would simply do only that if I had it to do over again. That ten inches makes a big difference.
If you only need seven inches, you can widen the berth by extending into the passageway.
Or you can do both.
Hope this helps.
Rick