Option 3 is definitely valid on the ticket, however I prefer option 1 instead.
I'm en route now.
I didn't want to do option 1 under time pressure and there's never any guarantee the ticket window will be open when you need it, so I set out a little bit early. Once I'd set out early there was little reason not to go the slow way and benefit from the cheaper fare.
I only began my journey at Abbey Wood today because of the train strike. Normally I'd start from Slade Green. For the future I've now worked out my best option is a Z1-6 travelcard in conjunction with a ticket from Boundary Zone 6 or West Drayton (doesn't matter which) to Slough (or Maidenhead, wherever I'm going). That has a number of advantages:
-no inconvenient routing restrictions,
-ability to break my journey anywhere in London, including LU stations, if I want,
-ability to take the bus to/from Slade Green at the start and end of the return journey,
-if there are major problems on Southeastern/Thameslink, ability to take the bus between my home and Abbey Wood instead.
Essentially, to somewhere in zones 1-4 without using the core section of the Elizabeth line.
Thanks for the responses.
It doesn't strike me as particularly reasonable for this restriction to exist at all. It is reasonable for passengers to think, as I did, that tube fares are either valid on the Elizabeth Line or they're not. None of the information made available to passengers says anything about the core section being different to the rest.
I don't believe, as a passenger, that I can be expected to comply with routing restrictions that aren't printed on the ticket, weren't drawn to my attention when I bought the ticket and aren't in the NRCoT.
To my mind there's an unexplained inconsistency between making a "London Underground Zones 1-4" ticket valid for travel to and from Abbey Wood, which is only reachable by NR and Crossrail, and at the same time saying that a routing restriction that only specifies "London Underground from Ealing Broadway" isn't valid on Crossrail to Ealing Broadway. There's no way for an average passenger to know that based on the information provided.