You're close to harvest? Let it go, start the flush and see her REALLY turn to shit.
I think you're worrying too much. I have leaves turn yellow and fall off all the time, even in veg, it happens to a few. You're not going to get these beautiful "perfect" looking plants with just a few grows. There's always something that isn't quite right...it looks ugly here and there, but it's all good in the end.
I'd just start my pure water flush now...nothing but water. She'll turn yellow, most of the leaves will fall off, but you'll get some tasty buds.
That's my opinion...others will give you other ideas...