I'd love to see this succeed, but I think it is a long shot, particularly given the current climate.
I had a look at this many years ago and history suggests that Congress intended the current situation where dependents are counted against the numerical limits.
This is from memory, but the essential facts are correct.
Back in 1990, the House and Senate came up with separate Bills with differing content, which then had to go through the Reconciliation process.
One Bill (the House one, I think), proposed an EB allocation of c.70k which explicitly did not count dependents.
The other Bill, proposed an EB allocation of 140k which explicitly counted dependents.
After reconciliation, the final Bill had an allocation of 140k for EB, without any explicit language either way.
It's not really credible that Congress decided to double the number of visas available to EB during the reconciliation talks, when each of their individual proposals resulted in broadly the same number.
The final figure strongly suggests that the intent was to include dependents, but the explicit language was missed in the redrafting of the hybrid reconciliation Bill.
I wish it were otherwise. It would solve virtually all the current problems of visa numbers.
Addendum
A pro piece
Why We Cant Wait: How President Obama Can Erase Immigrant Visa Backlogs with the Stroke of A Pen by Gary Endelman and Cyrus Mehta also mentions this situation (see section with footnote [8]).
The antis are certainly aware of it. I really don't like linking to an organization like this, but it explains the argument and has the actual figures in greater detail.
Could Obama Increase Immigration By Not Counting Family Against Visa Caps?
The articles are a bit of a read, but they do give the background to the issue.
It's certainly the counter argument that this lawsuit will come up against.