score:9
Since you pinged me on chat about this, I'll do my best to answer... But honestly, I think it's a very poor question. Therefore, I provide two answers: The one I think you're looking for, and what I believe is the proper answer.
I think an OEC who uses this verse as a proof text is using it in the sense that to God time measurement is essentially meaningless. They clearly don't mean to interpret 1 Genesis day as a literal 1000 years, but rather they use the 2 Peter reference as evidence that it was "an indefinite, arbitrary period of time, which to God could just as well have been one day."
In this sense, your first counter-point is invalid. What Peter is saying is not that 1 day = 1000 years, or 1000 years = 1 day; but rather he's simply making the larger point that "To God time is not a concern!"
In that sense, if God says he spent 6 days creating the world, those 6 days are immaterial, and could be any amount of time, or non-time, or whatever.
Time is immaterial to God
However, this leads to my second answer, which I think is more of the real answer. And that is that any honest, studied OEC will not use these verses as proof texts for their view, simply because Peter is not making the specific statement that one day is a thousand years. Peter is not providing a formula for interpreting other scripture. Peter is saying time is immaterial to God. If one is going to apply this verse to Genesis 1, they might also be tempted to say that the Battle of Jericho lasted 6,000 years.
So, an honest OEC would not use these verses the way you presume. When YECs "debunk" this verse as supporting the OEC view, they are debunking a straw man.
An honest, well-studied OEC has a lot of reasons to think the days in Genesis are not literal, but, at best, 2 Peter 3:8 is a "passing curiosity," and not the core of their evidence.
Having said that, there is a subset of Young-Earth creationists who do interpret Peter as providing a formula--perhaps as an attempt to lend some additional time to the creation story, to better fit with the scientific understanding.
I'm not sure this view is widely held any longer, and I doubt it was never a majority view. I think by now YECs realize that adding 6,000 years (1 day = 1,000 years x 6 days = 6,000 years) doesn't even begin to "help solve" the discrepancy between the scientific explanation of an old earth, and the YEC view of a young earth. It is easy to find examples of (mostly YECers) refuting the concept that creation took 6,000 years.