PumkinsAvatarTools VS VRCMelonAssistant

you can use Pumkin's Avatar Tools https://github.com/rurre/PumkinsAvatarTools

well theres no reason to unpack prefab, you can add things without doing that, and if you want to re-order stuff do it in blender if you want to transfer things from 1 avatar to another in the hierarchy, you can use pumkin tools https://github.com/rurre/PumkinsAvatarTools

Does https://github.com/rurre/PumkinsAvatarTools contain stubs for Dynamic Bones?

Use Pumkin Avatar Tools. It can autoremove the physbones with one button

I recommend using Pumpkin's Avatar Tools in Unity instead. It has a tool you can use to easily play animations on models among other things. I personally found a Fortnite animation pack that I use to test my weight-painting,

What could be happening is that the animation is still in "preview" so if you go back to the animation tab below and click the preview button, it should release that pose and return to a T-pose state. If that isn't the case like most people suggested using Pumkin Avatar tools can also help as well. https://github.com/rurre/PumkinsAvatarTools/releases

Instead, find the .fbx file (or OBJ) that came with the avatar and put that into Blender, then use https://github.com/rurre/PumkinsAvatarTools/releases/tag/1.1.1 to copy thing over from the already setup avatar. Remember that while exporting in Blender, disable it adding leaf bones, it is in one of the drop downs when exporting.

If it is Dynamic Bones related, I see that Pumkin's thing here has a button to remove missing scripts. "Fix Missing DynamicBone Scripts in Prefab" https://github.com/rurre/PumkinsAvatarTools

In case you haven't already, https://github.com/knah/VRCMelonAssistant Snatch some anticrash mods from here, and I suggest going into your safety settings and turning off everything for anyone that isn't on your friends list. I keep Trusted and Known user avatars shown, but keep their shaders, custom animations, etc off at all times. I've never been crashed.