You could probably just make a simple interfacing class that allows uSequencer to call SetAnimation and other things you need in the Spine API.
mixing will be done automatically in that case. The same can be done with Unity's own native Animator, although that can be uglier since Unity Animations use SendMessage.
Whatever the thing is, given what you need, you won't be animating cutscenes in Spine. You'll be doing them in Unity, and you only need to interface with Spine's regular API as you would in-game.
If you have the Unity Editor expertise and time, know that you could probably create a scrubable timeline with Spine animations inside Unity. On the Spine-Unity side, Spine skeletons can be posed given an animation and a point in time (float) so it's very doable.
