The facts are the facts. You don't have to be certified to bring that out. I am working towards ICC cert. Will it make me a better inspector? Sure, but even without the cert. I can still read a book.
I do a lot of Code work. The cert doesn't back up your word or report. The Code book and what it says does.
Forget about the builder and his BS. He pretty much has to BS you. The more he BS's you, the less it may cost him to fix all the things he already knows are wrong.
Your client and the attorney's aren't going to give a damn about 'his' demands when faced with copies of code pages for all the things you write up.
A suggestion, call the muni building dept., ask what Code is current for them now. If you have that great, if not buy it.
If you aren't comfortable (which it sounds like) doing this insp find another HI to do it with. Don't just let the lead go.
Good luck

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Accurate Inspections & Consulting, Inc.
773/844-4AIC
Markus Keller