What percentage of sleep paralysis incidents are caused by spirit meddlers and what is the mechanism?

This is almost entirely a consequence of spirit meddler interference. It is occasionally a discarnate human spirit and on occasion as well it can be an extraterrestrial incursion that is perceived on some level of the mind and the person has some waking perception but is unable to move because they are frozen in place to begin an abduction or to keep them immobilized during the exiting of the abducting extraterrestrials in the process of leaving the premises. The spirit involvement is by far the highest percentage, being 95%, and this is a consequence of their ability to manipulate the being and to gain control of the body itself for a time. It is often done to shock, to traumatize the individual, to raise fear, all of which serves onboard attachments drawing energy from their host. The influx of negative emotions ramps things up and gives them a surcharge of energy that will benefit these spirit parasites. It is sometimes done to satisfy inner urges of the spirit who wishes to mount an attack by presenting a threatening visage. This is often done by spirits of people who have been sexual predators in prior incarnations. This can be both male or female and is classified as the incubus and succubus, respectively. So the individual victim will feel unable to move, will sense something hovering oftentimes, and may see an actual being and feel manipulation of their body, including sexual acts done to them. This is always done in concert with onboard spirit meddlers who are exerting the control of the person, so it is not the outside attacker causing the paralysis, it is a tag-team operation, so to speak, where the onboard spirits commandeer the body to be immobilized and then the outside attacking spirit comes forth to add to the fear of the moment.