You pose too many questions to adequately answer on a forum such as this. As counsel suggests, it may be a good start to speak to a criminal defense lawyer regarding the pictures that are on the computer. Without more information, it is difficult to determine whether the pictures will cause problems with the OP&A. If the pictures are found to be sexually explicit and certainly the nature of the pictures (sexually explicit of children, disabled adults, etc.), then big problems will be coming. The fact that the files were password protected will not provide any type of defense or protection.
As to whether the OP&A is planning to challenge your guardianship, again, without specific details it is difficult to answer that question. Talk to an attorney.
Note: This response DOES NOT constitute legal advice and therefore no specific action should be taken in reliance thereon. No attorney-client relationship is created through this response. You should speak to an licensed attorney in your state who is competent to answer your question before taking any action with regard to this question.
We can not predict what they will challenge and what they'll do. You and your son should both get criminal defense attorneys. The attorney can decide if they had the necessary paperwork to do the searches etc. Ask your attorney.
We do not have an attorney-client relationship. I am not your lawyer. The statements I have made do not constitute legal advice. Any statements I have made are based upon the very limited facts you have presented, and under the premise that you will consult with a local attorney. This is not an attempt to solicit business. This disclaimer is in addition to any disclaimers that this website has made. I am only licensed in California.