Amphorde

They still do but not always, like before

The video shows the effect of knockback too, I tested against a wall (and with netherite armor for knockback resistence) and it was only visual as seen on this video :

That's intended, other mobs also have different damage outputs based on the difficulty and it's the principal thing changing the difficulty does.

I think this is Working As Intended because it's just that glow squids ("glow octopus") have only few spots to spawn because of the skyblock world so a lot spawn there (up to around 70 can spawn in a surival world I think and each time they die new ones can spawn)

J'ai vu l'autre reporte de bug en français donc je vais aussi traduire mon commentaire :

Je pense que c'est une fonctionnalité qui marche comme elle devrait car le problème est juste que les poulpes luisants ont qu'un seul endroit pour apparaitre à cause du monde skyblock donc beaucoup apparaissent et comme ils meurent en boucle la limite d'environ 70 créatures de ce type n'est pas atteinte.

It makes a gameplay difference as you can search in the creative inventory with the hashtag for a group of items :

I think this is a duplicate of MC-62104

There is no tutorial world in the Java edition

It is an intentional feature. Here is the changelog from the snaphot that says it : https://www.minecraft.net/en-us/article/minecraft-snapshot-22w45a

Vex

• Changes to Vex model and textures

  • The Vex retains a slightly oversized hitbox to make it easier to fight

This is simply one of the new default skins added, the game doesn't detect if you are a girl or a boy it just give you one of the skins randomly I think

Did you put the datapack with the 1.20 experimental features when creating the world? if not then you cannot have access to them