I have problem with Mixamo animation download

the weapon had a bone Mixamo was reading as an arm bone... you could add it back on now that the model has its Mixamo rig or you could add just the mesh without the bone

the distortion is the weight map; the left arm is different to the right arm and the head / helmat has no neck... both are outside what Mixamo is designed for and easy to fix in Blender or whatever animation engine you end up using

Mixamo will not touch a rig if it detects bones, it will only add bones to a blank model... the idea is you then edit the rig for what ever extras you want

yes if you want to share the model i'll take a go at fixing it

i'd put him in t pose then remove all the bones and save that model as a test for upload