According to your code and also verified this in game your king spider doesn't remove passengers before adding your druid. Since it's not forced on the entity the druid doesn't spawn in at all and all that remains is a regular skeleton. Not sure if this is intended but, It seems like a bug to me.

what to do instead. Remove passengers before creating your druid and placing it on your king spider

KingSpider#onInitialSpawn

Oh yeah I'll probably do something evil with this

Worth noting: https://bugs.mojang.com/browse/MC-114019 (Since i know what you're on about)

But ya, this sounds like a feature to me :^)

has nothing to do with mob spawners. Research before you comment. This can occur regardless of how the entity is created on server side.

I did research and I know how you discovered this. I'm just throwing it out there because I know exactly what you're trying to do in your own mod.