I don't personally think that Porter's car needed to have been at #35 that morning, for an Abbott theory to apply.
Geoff Owen was in Kendall that morning. He could have dropped Abbott off somewhere close by.
To me, it is not unlikely that Abbott carried William to the school. Abbott was accustomed to walking a lot. And if Porter didn't arrive there till after 3pm, Abbott had a lot of time with William
before Porter arrived.
If Abbott did carry William away, it could be why Savage disappeared for 1½ hours to see if he could see where they went/find William.