Character model rigging error

I am currently porting a model from Grand theft auto 5 for use (Non commercially) in VR chat, I have previous experience with some models before but those were much easier to do as they were in perfect T pose.

Anyways the issue Im having here is this model when put into unity and in the bone configuration area, the shoulders are a bit off and I cant seem to figure out how to edit the mesh/bones in a way where it can perfect T pose without the shoulders being distorted in this way.

The model isnt in a perfect T pose as you can see from the blender screenshot and manually editing the models arms to be level isnt a very feasable thing to do (with my current experience and knowledge) the mesh isnt completely joined together (despite technically being just 1 mesh) the arms are not connected to the shoulders for some reason Im not sure why this is so but, I guess its a model made by Rockstar games soooo I guess I cant argue.

Anyone know a way that I can fix the shoulders so he looks normal while in perfect T pose form, the rest of the model is rigged great its just the shoulders that are having a problem really :L

Ive tried rotating/moving the shoulder bones around to see if it would make a difference when ported to Unity but every single time no matter what I do Unity always forces the models shoulders and shoulder bones in the positions as pictured.

Thank you for reading and hope you can help please thanks!

 

I'm no pro at this stuff, but I've had to deal with janky rigs and what I do first is upload the model to Mixamo and see if their auto-rigger can fix the problem. If that doesn't work, I just find another model. Sorry I don't have anything better to add, but I thought I'd at least tell you about Mixamo. It's free to use.