How Can you fix Avatar Masks and Mixamo Animation bone Structure for Unity?

Question

I'm having a bit of a kerfuffle I have a Character in my Unity game which cannot use Avatar Masks with an animation from Mixamo. If you don't know Mixamo, i'ts a website for downloading pre-made animations, which you can use on your own models or their's, and import and use them in various 3D software, including unity I just learned about the Humanoid Avatar definition (I will refer to this as HAD), and noticed that my Human character's bone structure "which uses a Mixamo bone rig" is not completely following this. The Avatar mask in unity will only work, if the HAD is not followed.

The Structure goes as follows:

|- Hips
  - Spine
    - Chest
      - Neck
        - Head
          - LeftEye
          - RightEye
          - Jaw
    - LeftShoulder
      - LeftArm
        - LeftForeArm
          - LeftHand
            - LeftThumbProximal
            - LeftThumbIntermediate
            - LeftThumbDistal
            - LeftIndexProximal
            - LeftIndexIntermediate
            - LeftIndexDistal
            - LeftMiddleProximal
            - LeftMiddleIntermediate
            - LeftMiddleDistal
            - LeftRingProximal
            - LeftRingIntermediate
            - LeftRingDistal
            - LeftLittleProximal
            - LeftLittleIntermediate
            - LeftLittleDistal
    - RightShoulder
      - RightArm
        - RightForeArm
          - RightHand
            - RightThumbProximal
            - RightThumbIntermediate
            - RightThumbDistal
            - RightIndexProximal
            - RightIndexIntermediate
            - RightIndexDistal
            - RightMiddleProximal
            - RightMiddleIntermediate
            - RightMiddleDistal
            - RightRingProximal
            - RightRingIntermediate
            - RightRingDistal
            - RightLittleProximal
            - RightLittleIntermediate
            - RightLittleDistal
    - LeftUpperLeg
      - LeftLowerLeg
        - LeftFoot
          - LeftToes
    - RightUpperLeg
      - RightLowerLeg
        - RightFoot
          - RightToes |

I noticed the Legs in my character is not inside the Spine bone under my character, which in short breaks the HAD. I tried moving them inside the Spine bone/object, but then the animation itelf will break, which makes my character look incredibly stupid when he is running: (Visual of the Running animation playing)

Is there an easy way to be able to fix this? I can think of a hard way, spending hours recreating fbx-files with the bone structure correct, but in the process break animations and try fix them countless times, I would assume there was an easier solution than having to open Blender.

Information to note: -The animations are Read-only in the unity animation window -The animations are taken from FBX-files downloaded from Mixamo -The Bone structure for the characters two legs lies outside the Spine object/bone in the hierarchy, which breaks the Humanoid Avatar Definition, which an Avatar mask relies on -The Bone structure for my character is exactly the same as a downloaded rig from Mixamo