Contrary to what OP asserts, BC's are not easy to forge. I am not referring to cutesy stuff which people print for fun, but the Certificate of Live Birth issued to the parents/guardians. It also follows that most people who might commit a petty fraud against an air carrier, won't commit a federal felony with a BC.
If people weren't crooks, FAA would have the requirement and WN (and other carriers) wouldn't have to be so tough about it. There's a real temptation by people to save the $ and put their kid at risk at >2 and the only way to be certain is is to always require it.
WN ought to accept a passport (most carriers do), but WN can be WN and when its agents get their backs up, they get their backs up. I would call WN and ask whether a Passport will suffice and to have the PNR notes reflect this if it does.
Not directly relevant, but there is a decent amount of research suggesting that a kid in a car seat is a lot safer in turbulence (common) or a crash (uncommon). So, maybe safety trumps here anyway.