Suppose for the sake of argument that a publisher is not compliant, because producing material that is ipso facto OGC (due to (i) its relationship to OGC in the SRD, and (ii) the operation of the OGL) but not declaring that under section 8.
Who is going to sue? WotC could if they wanted to, because the publisher is not compliant with the licence. But WotC probably don't care, because WotC has shown very little interest in reproducing OGC created by other publishers (I know there are a few exceptions, but not many). It's not as if the publisher is using content of WotC's that it couldn't gain access to in a compliant manner.
Other 3PPs who might want to use the non-compliant publisher's OGC might be frustrated that the publisher is not compliant. But unless the non-compliant publisher is using that other 3PP's OGC, I don't see that the other 3PP has standing to sue. Perhaps they could use the non-compliant publisher's non-declared content, and then if they were sued by the non-compliant publisher they could try and set up the non-compliant publisher's contractual obligation under the OGC as a defence - I don't know if that would work in American law, but I don't think it would work in Australian or English law.
In other words, if WotC doesn't want to use other's OGC then it has no incentive to enforce section 8 declarations. That is something for the 3PPs to sort out among themselves, and they are more likely to do it in the informal ways that you describe rather than via litigation.
As to whether evidence of good legal advice is evidence of compliant section 8 declarations - I don't think that's a sound inference. What's the worse that happens if your section 8 declaration is non-compliant - a rectification notice with 30 days to cure breach? If you don't want your material to be OGC (because you want to maintain control over your IP) and you think there is a least an argument that it is not OGC (the notions of "derivative work", "addition", "upgrade", "extension" etc are hardly cut-and-dried) then there is good reason to have a go and see how WotC responds.
As it turns out, WotC isn't very litigious. (They let OSRIC go after a polite inquiry or two.) Probably for good reason - litigation is expensive and unpredictable.
The weirdest OGL thing I've ever come across is that the PF Alpha, PF Beta, and PF SRD (and therefore, presumably, PF? I don't have any PF books other than the Alpha and Beta) all contain elements from the 3.5 SRD (eg Polar Ray) and yet in the section 15 declaration only mention the original SRD (the 2000 one) and not the 3.5 SRD (copyright 2000-2003, and with a larger list of contributing authors). I don't know what's going on there.
