The cover quotes and flavour text in manga magazines are usually done by editors, not the manga's authors, so that's why it specifies it's from Itagaki.
I can't remember if he died. I feel like they said that in Yuuenchi but I can't even remember if that's accurate, never mind if it's based on him dying in Grappler Baki.
Refer to the title. Baki Rahen roughly (and supposedly) means "those around Baki", Itagaki is definitely wanting to shine a light on more shunned characters with this part. Jack having a major role with his own arc kind of threw people off of that.