How should one address negative circumstances with past advisor in a Statement of Purpose? I am referring to a situation described in a previous question of mine:
How to deal with an abusive advisor?

To summarize for those who don't want to read the lengthy story: I had a difficult relationship with my previous Master's advisor, who was psychologically abusive towards me. I was also dealing with (unrelated) clinical depression at the time, which affected my productivity.
Eventually I decided to switch to a different advisor in an unrelated field. I am happy with my new advisor, and am being more productive, and the split from the previous advisor was on decent terms (that is, he still had a bad impression of me as a student, but he supported the decision).
Due to the previous issues and the switch of field, my degree took twice the "standard" time to finish (4 years instead of 2, although at my university/in my country even the top students often don't finish in 2 years).
In addition, my previous advisor's name will appear on my graduate transcript (along with the period of time during which he was my advisor).

In my PhD application (specifically, the "Statement of Purpose") I will probably have no choice but to address this issue. Since there is no option for anonymity, the story is not just about me, it is an "accusation" against him, which means the committee might want to hear his side of the story, in which case it will of course not go in my favour. Also, while he is not very famous, he is friendly and popular with colleagues, so if someone knows him personally they might simply not believe me.

What should I do? Should I actually mention it? For those of you who are a part of admission committees, would something like that be a red flag? Would you investigate further, or take it as a reasonable explanation given positive letters of recommendation from other professors (and current advisor)?

Also, is past depression which caused lack of productivity considered as a red flag?
Congratulations on the switch, I'm so glad to hear your update.

There will be an ongoing need to heal from the trauma you underwent.  The SOP will be separate from that.

If you are concerned that an admissions committee might raise a red flag for the extra time you spent in your program, you may address that briefly in a cover letter.  For example, "You might notice that I spent longer than customary on my degree.  That was due to some health issues exacerbated by an uncomfortable advisor fit."  That's enough.

You might also alert your new advisor to your concern, so that s/he may choose to consider this when writing letter of recommendation.