How do I stop antler crown handles from splitting when I peen the pin?

The smell is normal, that's just the marrow and residual blood in the core of the antler burning off with the drill friction. Every bit of antler I've drilled stinks like that. It doesn't necessarily mean the piece is wet inside, though if the middle is genuinely spongey and soft then yeah, that chunk prolly wasn't fully seasoned. Antler takes a long time to dry all the way through the core, especally a big crown. If it was fairly fresh, you'd want a solid year or more in a dry spot before it's stable enough to work. As for the split, the peening is almost certainly the culprit, not your drilling. When you peen a rod end you're hammering that force straight into the surrounding material, and antler has basially zero flexibility in tension. It bulges and cracks exactly like you described. On the spongier core especially, the rod can mushroom underneath the surface and push outward before the end even looks peened. That's a very common thing with crowns and it catches people every season. A couple of ways around it. Leave the rod a bit short so the peened end sits slightly below the surface, then fill the recess with epoxy or a brass dust and epoxy mix. Or skip peening entirely and just rely on a good gap-filling epoxy on everything, rod included. With a crown that's already on a pinned tang, the rod is mostly there for alignment and shear strength anyway, the epoxy does the real work.

For the hairline crack you've got now, don't toss it. Work some thin CA glue into the crack, wick it in good, clamp or wrap the crown til it sets. CA does a great job stopping a split from creeping further and it's invisible once you've sanded and polished over it. If you can catch it with a thumbnail, hit it with fine paper first so the glue has something to grab, then seal the whole crown with a light coat of super glue as a finish. It'll look fine and the crack will stay put. And keep that piece of crown for a smaller project where you don't need to pin through the middle, the dense outer bone on a crown is beautiful stuff.

Agree on the peening being the killer here. I stopped hammering rod ends in antler years ago after cracking two crowns in a row. Now I countersink the hole a hair and stake the brass with a small center punch instead, little taps, working from the outside in. Spreads the force instead of driving it all down at once. Also I drill my antler holes one size up and go slow, like half the speed I'd use on G10, because that spongey core will grab a bit and tear the walls out on you. The split you've got will probably disappear under sanding dust and glue anyway, crowns hide a lot.

This makes me nervous about a chunk of antler I've got sitting in my garage. My uncle gave me a couple of sheds last fall and they've just been leaning in a corner since. Sounds like that's way too soon to drill into them? Also curious how you tell when one is ready, is it just a weight thing or do you tap it like a watermelon or something.

I'd take the CA fix with a grain of salt on this one. Thin CA wicks great into dry bone, but if that core is still spongy like he said, there's moisture in there and super glue doesn't cure right against damp material. It'll look fixed for a week and then let go. I tried sealing a crack in a crown I'd rushed once and it went cloudy and soft under the glue. Let that chunk dry out a good while first, then glue it. If the crack has moved by then it was never going to hold anyway, better to know.