What stage of dementia do you think she has? What should be done at this point?

It sounds like Mom may be well into dementia. The "stage" is not really meaningful. Different forms of dementia progress differently, with different symptoms coming earlier or later. Since you have no idea what kind of dementia she may have, talking about stages is not really very useful.

What should be done at this point is she should be seen by a doctor, preferably a geriatrician. If she has a good relationship with a primary care provider that is the place to start. Dementia is not curable, but there are treatments that can help with some of the symptoms. For example, I am not aware of anything that helps with the repetitions, but there probably is some help for the screaming for hours on end. Get her to a doctor.

Also, educate yourself (and, gently, gradually your father) about the most effective ways to deal with persons who have dementia, and about what you can expect as it progresses. There are many good books on the subject. Lots of personal experience is shared on this site. Do searches and see what others say about dealing with someone who repeats herself over and over, with someone who won't bathe, with screaming, and with each symptom you mother may display.

Good luck.

Here are the 10 Warning Signs of Alzheimer's and Dementia:
https://www.agingcare.com/articles/alzheimers-disease-dementia-warning-signs-144253.htm

1. Forgetfulness and memory loss
2. Lack of concentration and confusion
3. Losing things
4. Difficulty doing familiar tasks
5. Language and speaking problems
6. Problems with simple math
7. Poor judgment
8. Personality changes and mood swings
9. Changes in grooming and personal hygiene
10. Withdrawing from friends and family

Hope this helps :)