If the frequency does lower abruptly due the redshifting, could we tell the difference between that and it just stopping?

The waves come from the circling, not the merge itself.

Yeah… But the singularity is hypothetical, we don’t know if they exists. We know that blackholes exists, but how their inside’s are is a different topic. And things don’t just appear to be frozen when they approach the blackhole, they (from our perspectives) are frozen, they did not crossed it yet because of the time dilation.

That will demand a direct observation to answer

No. The escape velocity from “inside” the black hole is higher than the light speed. So, even if something happens “inside” the black hole to generate gravity waves, those waves will not be fast enough to escape.

But more important than that, the blackhole (IMHO) is an event that is happening, there is no “inside” yet, because the body that originated the blackhole is frozen in time, from an outside perspective, there’s no singularity yet because as density goes up, the proper time from this object slows down infinitely close to zero. If you wanna know more about that perspective on blackholes, look for “frozen stars”.