The "Mein" in "Mein Kampf" for the title is written 魔淫 before the 姦婦 (kanpu/paramour) part.
Mein Kampf is the name of the doujin if you go by the hiragana/katakana portion of the title. If you translate the kampf part as Paramour, you're not translating the mein part.
Of course, かんぷ might be in katakana because it's supposed to stand out against the まいん so there's also that.
In the end, it's a pun that doesn't translate.
(Also, I would like to note that, after doing a bit of research myself, マインカンプ is indeed the way how Japanese people refer to Mein Kampf if they were to use the katakana. Of course, they would probably use 我が闘争 (わがとうそう) if they were to actually refer to it more often than not.)
So don't get too caught up in it and just go with whatever name you prefer, I suppose.
As naming goes, it's "My Paramour" if you strictly use the English viewpoint of naming. If you know Japanese and wish to incorporate the japanese language homonym pun, then you use "Mein Kampf". If this doujinshi were for common western consumption by the masses, then "My Paramour" is most appropriate and it even relates to the topic of the doujin. Generally, overly literate untranslatable puns such as "Mein Kampf" do not fare well with the common consumer.