FWIW, here's my take on all of this...
I am not a lawyer. I don't hire one to go over my products and make sure they are OGL/d20-STL compliant.
I do make sure they are OGL compliant the simplest way I know how...
Step 1: I grab the appropriate sections of the SRD. (I NEVER crack open a WotC book like the PH, DMG, or MM when I'm writing stuff). I create a new file and copy/paste the relevant sections into the new "working file".
Step 2: I grab stuff from other works that is relevant PROVIDED I CAN CLEARLY IDENTIFY IT AS OGC FROM THEIR DESIGNATION and immediately drop the appropriate info into the Section 15 of my product. As an example, I might pull text from Bastion Press' "Minions" book "since the entire text of [the] book" is designated OGC and I therefore know I'm safe. I then drop the appropriate text into the "working file."
Step 3: If there's something that I would like to include that I can't identify for sure is OGC, I send off an e-mail to the publisher that quotes the material I want to add and ask (a) "is this OGC" and (b) "if it's not, may I have permission to use it in this manner: <explains product idea>." When the publisher gets back to me with an affirmative (I say "when" because I have yet to see a d20 publisher say "no" if you make a reasonable request) - and ONLY then - do I add that material into my "working file" and update my Section 15 immediately.
Step 4: I do outside research as needed and put the appropriate information into the working file (e.g., for the Ench. of Treasures and Objects d'Art, I have researched object densities, Moh's Hardness, and other facts which cannot be copyrighted). Because this research is interested only in facts, which cannot be copyrighted, I do not need to do a Section 15 entry (or otherwise) here. I am always careful to keep my facts very short and simple so that I can't be accused of copyright infringement (nobody can sue me because I copy/paste "The Hardness of Diamond on the Moh's Scale is 10" into my working file because it is a statement of fact - though I'm more likely to have a simple table that has "diamond" in one cell and "10" next to it).
Step 5: I now write the book, using ONLY stuff in my working file for reference. Because everything in there is either OGC, stuff I've gotten permission to use, or simple fact, I am now 100% okay OGL-wise.
Step 6: When I am finished, I do a search on all of the "change is forbidden" terms per the d20STL. Since I am already OGL compliant, that's done. If I have not changed any of the forbidden terms, that's done. All that's left is to make sure the logo shows up on the first page of my document and that I have the correct "This product requires..." and "blah is a trademark of WotC" lines inserted into my document.
You don't need an IP lawyer to do it this way, and since you're using 100% "safe-harbor" material, AFAIK you can't go wrong. But if I have gone wrong somewhere, I'm sure I'll be told so.
Seriously, though, if you're going to write material for public consumption, following this formula is a nearly foolproof way to avoid getting in trouble... though knowing me, I'll be the example of the fool in the classic "make it foolproof and someone will invent a better fool."
--The Sigil