Need advice on attribution/copyright of heavily modified OSS code (BSD, Apache, etc) in source headers [closed]

Answer 1

The BSD family of licenses expressly forbid you from modifying or removing their headers.

Edit: I suppose I should go more into depth on this.

Essentially, you used their code as a basis for your own. Due to that influence, you are still bound by the original license, and thus have to keep the original attribution intact.