I have not read any good, scripturally based analysis here that proves the wicked servant is an unbeliever.

The Lord would not call an
unbeliever His servant and entrust Him with
His household, and neither would an
unbeliever call the Lord "My Master" and believe
He is coming back. This is unscriptural, unfounded and silly. You cause the parable to be
devoid of its meaning. The purpose of the parable is to teach
believers about faithfulness.
Yet, I wonder what is the reason behind this inclination to alter the meaning of a simple parable that teaches faithfulness or the lack thereof, between the Master and His servants, the Lord and His believers. So rather than belabor the point now, maybe itīs good to consider that there might be a reason that runs deeper than theology or biblical interpretation.
The reason could be pain....scars....hurt; thus a resistance to these truths, particulary the "
discipline aspect" as may befall believers, but because of the abusive way these truths have been used by some in the LR and the pain that causes. It is abusive when these guys use passages like this one and hammer you about 1,000 years in outer darkness and about not being an overcomer,
yet associate it with "not being one with the ministry", not going to all the trainings and meetings, using non-standard LC attire, listening to non-LC music, growing beards, too much time with family, not serving enough, leaving the LC, going on vacation, having a TV, reading other christian authors, etc. etc., ad nauseam.
But donīt throw out the baby, ...just get rid of the dirty water. Years of hearing these kind of beatings do take a toll on anybody, they become a heavy burden and weigh you down. But the Lord is rich in grace and healing. He knows, He was the target of the same religious beatings and much worse. But He still admonishes us to be faithful to Him. We were bought with a price and He desires a faithful Bride. This parable is for the benefit, instruction and hope for believers. Though we should not desire bad things happening to people, it is good to know that justice will prevail in the house of God. Vengeance belongs to the Lord. Those unfaithful believers that have been beating others unjustly in all kinds of ways and those that have improperly "covered them" will have to give an account unless they confess and repent. They will be disciplined.
Those that remain faithful will be richly rewarded.