I just read the memo. I think it is very clear.
1. After you enter into USA with AP, you are in parole status.
This is explained in question 3 itself, not the answer. The question 3 says "If an H-1 or L-1 nonimmigrant has traveled abroad and was paroled into the United States via advance parole, the alien is accordingly in parole status." It states a fact that after using advance parole, you are in parole status.
2. You are allowed to extend the H1B.
This is explained in answer for question 3. One more thing is that extending H1b will terminate the grant of parole, I would think it as converting you from parole status to H1 or L1 nonimmigration status rather than invalidating the AP document. But to be safe, you can just get a new AP before you leave USA.
3. You can work for your original employer without obtaining EAD.
This is explained in answer for question 4. "adjustment applicant's otherwise valid and unexpired nonimmigrant employment authorization ...is not terminated by his or her temporary departure from the United States, if prior to such departure the applicant obtained advance parole" Your nonimmigration employment authorization (H1B I129) is not terminated means you can work based on it without having a EAD (I would think it is an immigration employment authorization).