Disagree, putting any bukkit-based code on Forge is almost guaranteed to be a instant copyright infringement (remember it was DCMA'd not long ago, Spigot is just working in gray area to keep it updated).
Forge itself is good enough for small physical server-only mod mods, and if you want to stack another solution on top of Forge (just to feel more dangerous to run a server) use Sponge.
Attained Drops is broken and needs to be fixed by the author.
Error comes from Forge, because AD's CachedIngredient doesn't override a required method.
Known - there is a pull request (#5490) in progress for both those issues. Not sure how long it will be before you will see it in the beta version available.
I know people like me have already zadolbali, but is it possible to make the kernel support Spigot? After all, in the server folder all exactly is minecraft_server_1.12.2.jar why not do the same only with spigot-1.12.2.jar? Or suffer and do as in KCauldron one fine file. And don't write me about the sponge!!!! IT'S NOT CONVENIENT AND IT DOES NOT HAVE THE NORMAL PLUGINS.
Not sure why you're bringing up kernels...
If you want Bukkit/Spigot support you can write it yourself if you know how.
There will be no official support for Bukkit/Spigot and you won't receive support in this venture.