An update for anyone who's interested. Unfortunately, not really any good news.
It's been roughly 4 weeks since the first neck was sent back and I haven't heard anything so I gave Warmoth a call. Joe is the CS rep who's been helping me with this thus far.
In short, outside of an initial assessment, the neck hasn't been worked on yet and they were unable to provide a rough ETA for when it might be fixed. The neck originally only took 2 weeks to be received from when it was originally ordered (showroom item) so that was disappointing to hear. I had hoped that resolving issues with preexisting orders would take priority in their workflow but I was corrected that that is not the case. They recommended I follow up every two weeks until work is complete. Not thrilled about that but it is what it is.
On a more concerning note, when we got around to talking about the backbow in the first neck they said that they didn't find any when they checked it. To be blunt, that was utterly baffling for me to hear because the amount of backbow present in the neck wasn't at all subtle or hard to find. I'd even sent them photos of gauged measruements demonstrating the issue and they refused to acknowledge any of it. They just kept saying things like the "neck is fine" or the "truss rod is in good working order" (which isn't at all the issue here) and that the neck was only marked down as being in for fretwork fixes and that was it. They otherwise completely stonewalled me on it. The only thing that caused them to pause and backpedal a bit is when I asked for them to send me a photo of the neck with a notched straight edge on it, showing me that it was either flat or had some amount of forward bow, at which point the excuses started pouring out.
Unfortnately, the call ended without them agreeing to any concrete action. Just that I could send it back again if it still has issues. They weren't even willing to say that they'd check it again just in case.
So I'm betting I'm going to receive it back with the exact same issue. Absolutely wild to me.