1. What is the most current Systems Engineering Guidebook Version and date?
2. Where does it reside (url)?
3. https://acc.dau.mil/oa is no longer valid. Is there a new url (or is that answered in question 2)?
Note DAU's website have been updated to end with .edu instead of .mil. Given recent changes to the acquisition policy eco-system, it is recommended to review DoDI 5000.88 dated 18 Nov 2020. It incorporates the current DoD policy for engineering of defense systems, to include systems engineering.
The guide for SE is contained in Chapter 3 of the Defense Acquisition Guidebook (DAG), but that is to be updated in the near future to a "more modern set of acquisition guides" per direction from the last Under Secretary of Defense for Acquisition and Sustainment, Ms. Ellen Lord. Go to https://www.dau.edu/tools/dag for the DAG.