Why not just build a brace that goes underneath to support the rail from below the deck?

That way it is supported by the deck and it doesn't matter if anyone steps on it.

Build one for the area by the steps to begin with and if that works, then just build more for the rest of the deck area. Sure, without it being perfectly level between the deck and the rails it is a slightly more complex solution, but in the end it is the same.

And you cannot blame the installer of the replacement pool, since unless you replaced the pool with the exact same model you had previously there is no way to get it all to "match" without 4x the time and effort. You are talking about test fitting, tearing it down, re-leveling, fit it again, etc...

I would say they did a hell of a job getting it that good. And the extra expense was for them to get it to match exactly like they did. That is a huge effort in itself to get it that good.