MIMF

My limited experience with locust--Honey locust is not super hard, perhaps a little less dense and hard than ash. Black locust is quite hard and dense.

Which ever it is, cut it and keep it.

When I have kept woodwind billets and not boards, I used a wedge or froe and maul to split them out. Following the grain gave me pieces that turned (literally; as in on the lathe) out like I wanted even though they often looked wrong at first. As far as glue and sealing, you can use TiteBond II diluted about 50:50 and it works as well as AnchorSeal (cost is about the same; this is not a budget decision, just what to do if you can't find AnchorSeal) except it is tougher on tools later. TB I is the same as the Elmers and won't work; it has to be TB II (I have not tried TB III).

Auto anti-freeze will prevent and kill fungus (and pets and kids, so use care). It will wash out of wood if left exposed to rain. I've successfully used it to protect wood for boat building. I don't know if it would have any detrimental effects wood for instruments.