How do you keep brass wire inlay from lifting out of the wood over time?

The wire is mostly held mechanically, not by the glue, so I wouldn't lose sleep over the CA. When you hammer wire into a v-groove, the sides of the groove pinch it and the peened ends lock it front and back. That's the whole system. The adhesive is just there to stop the wire from lifting between contact points, and thin CA does that job fine because it wicks under the wire instead of sitting underneath it like a bed of epoxy would. In fact if you'd filled the groove with epoxy first, the wire probly wouldn't have seated down flush, you'd have had a glue layer pushing it proud and gaps everywhere on the curves.

Where epoxy earns its keep is when the groove is sloppy or the wire is loose in it, then you want something with body. Since yours sits a hair under the surface and the ends are peened into the bolster, you're in good shape. If a section ever lifts in a few years, a drop of thin CA and a fingertip presses it right back down. That's a two minute fix, not a failure. One thing to check on future work: undercut the groove slightly. Cut with the v-tool leaning a touch so the mouth of the groove is narrower than the bottom, and the wood actually grips the wire as it swells and moves with humidity. A straight-sided cut holds less. Walnut is pretty stable stuff so handle flex won't be much of an issue, it's not like inlaying into something oily or soft. On the sheath, the darker halo around the thread is completely normal. Wax brings out the color of everything it touches, and it soaks in more around stitch holes because that's where the leather is most porous and you worked it with an awl. It evens out a lot with handling, and honestly some contrast around stitching looks good, it reads as deliberate. If it bugs you, give the front panel a buff with a soft cloth or a piece of scrap denim and it'll knock the darkest edges down. Next time, test your wax on an offcut first so you know how much it'll shift the color before it touches a finished piece.

Interesting reading this as someone who only ever done that kind of wire work on a flat box lid, so the whole curved handle thing kind of blows my mind a bit. How do you even keep the grooves a consistent depth bending around the swell of a grip? On flat work I just let the v-tool ride and it came out fine, but I imagine the angle changes constantly on a handle. Also curious what you peened the ends into exactly, is the bolster drilled or do you just mash the wire into the pin heads?

Did a similar wire job on a pooled-handled skinner a few years back and the only spot that ever gave me trouble was right where the palm sits, the highest flex point. One section crept up maybe a year in, hit it with a drop of CA and pressed it down with a fingernail, hasn't moved since. Yours being peened into the bolster sounds even more solid than mine was.

On the sheath, I got in the habit of burnishing the stitch line with a slicker before any wax goes on. Sealed holes get way less of that dark ring. Live and learn on my end, your halo will probably fade into the rest anyway once it gets handled.

Update: the point about the groove sides pinching the wire settled my nerves, so I went back over the whole handle pressing along the line with a burnisher and everything feels seated tight. Still bugging me is the sheath, the dark halo around the stitching has not faded after a week of handling. Should I have burnished the thread before waxing, or will handling wear it in?