Is the tester an end user with the disability concerned (vision loss and/or fine motor control)? If not, I'd probably make the argument that how it is with the next button being the first option is best, because it eliminates excess tabbing by having the most frequent use button first. (ETA: If they are an end user with access needs, then I'd defer to them what is best).
Another option would be to remove the navigational controls from the player, and build in navigation buttons in your slides, which you could then set in the slide tab order.