No, it can't be inset at all, because the fencer will screw the pommel/insert down onto the grip (and the shim washers) as hard as their hand can, so the insert has to be flush to protect the plastic from pressure that could break the insert loose. The fencers want the pommel/grip/bell-guard/blade clamped down as hard as they possibly can, so nothing comes loose under the impacts of vigorous fencing. (I also recommend a drop of Loc-Tite. On my first metal pommel design, fencers griped because I didn't include flats for a 3/4" wrench to tighten it.)

I set it flush when melting it in, by pressing the base of the pommel with still-hot insert down flat on a metal table. It's in there forever after the plastic cools and shrinks.

The straight portion at the top of the hole should just be extended the 2.3mm at same diameter, upward into the cavity, to allow the threaded tang to protrude into the cavity unsupported. Many of the threaded tangs will extend further in than 13mm, and need to do it without hitting anything and breaking plastic. The uppermost couple of mm of the 15mm tall "plug" is there to spread the stresses from the end of the brass insert into the cavity walls.

So far none of my prototypes have broken in use, even when using brittle PLA plastic, but I take my design guidelines from the Navy: Design it strong enough. Then make it three times stronger than that...