Why is she called "Pilegesh Avraham"?
'Rashi': The Sefer says any possible detriment about Bnei Keturah 1 for Yitzchak's honor. The rest were Bnei Pilegesh; he was primary, and the [sole] master of the household.
Radak: The Torah said "va'Yosef Avraham va'Yikach Ishah u'Shmah Keturah." Here it informs that she was only a Pilegesh. Only Sarah was Eshes Avraham; Hagar and Keturah were Pilagshim. Also the Torah calls Bnei Keturah "Bnei ha'Pilagshim"!
Malbim: She was truly a Pilegesh. This is why her sons are attributed to her - "Bnei ha'Pilagshim"! Hagar was a full wife - "va'Titen Osah l'Avram? l'Ishah."
This implies that really, she was a proper wife, like it says "va'Yikach Ishah u'Shmah Keturah"; she is called "Pilegesh Avraham" based on her initial status. (PF)
Why did the Sefer omit "Ashurim u'Letushim u'Le'umim" (Bereishis 25:3)?
Malbim: This supports Targum Onkelos there, that they are not nations. Rather, such were their professions. Rashi said that they are nations.
