Hi. I don't play with this activated at the moment. Pretty sure I tested it with looting and it worked fine back when I played with it last year. Could be an update broke it or you have something else installed that affects the drop rate.

All mob drops and enchants should be working as in vanilla. Only way I know to change the chance is edit the loot tables for all entities separately.

If you want you can edit the files and change in each file the following line
"chance": 0.05
to
"chance": 0.5
That would make it drop at 50% chance. It's like 67 files to change tho :)

I went with 5% after playing with different values for a while and 5% seemed right to me.

Didn't know about that feature. Thanks for the info. Not sure if it's possible to disable it in a datapack.

<edit> Doesn't look like it is. There's a feedback request for it on the Minecraft website in case you want to vote for it. https://feedback.minecraft.net/hc/en-us/community/posts/4406263935757-Add-a-new-SpawnerOverride-tag-to-Spawn-Eggs