The Covid 19 vaccine could very well be the culprit for your dad's troubles. If you know anyone that works for or is working with a nursing home, make a call and they will tell you very quickly of the things they've noticed with the elderly after the vaccines. The 1st shot affected them some, but the 2nd one REALLY opens things up, especially the ones that are already suffering from cognitive issues. Delusions, hallucinations, memory loss, ect. is very common. With all that said, there are several other things that can also be working on your dad, many of which were mentioned above. Make sure that they checked him for UTI's. Brain scan, which sounds like they did. On October 9th we found out my mom had metastatic melanoma, had gone to her brain. She was fine on Friday morning and by Friday nite she was having cognitive issues, never was able to get any better cognitively. Lost her Dec.8th. My dad and older land lady was in nursing home during all of this, so had lots of interaction with them for the last year. Both had the 1st shot, dad passed on Feb 20th, so he just missed the 2nd one. Older land lady got both and had MULTIPLE symptoms that you describe. Many others in the nursing home suffered through the same things. Good luck.