Why does the silver bezel shadow show through my dendritic agate?

The grey halo is just physics, unfortunately. When the stone is translucent, you're seeing the inside of the bezel wall through the edge of the cab, and sterling at that angle reads as a soft dark grey rather than a crisp line. It happens with agate, chalcedony, moonstone, anything with body to it. A taller wall will only make it worse, since there's more metal for the light to pass through before it gets to your eye. Same reason the floor scratches show up, the stone is basically a little window and everything behind it is now part of the view.

Yes, polish the inside of the bezel before you set anything translucent. It's not wasted effort at all. Scratches and tool marks on the inner wall scatter light and make that halo fuzzier. I hit mine with fine grit paper wrapped around a dowel, then a quick tripoli, before the stone ever goes in. Same for the seat and the floor of the setting. Takes ten minutes and it's the difference between a clean edge and a murky one. For the wall itself, you've got two routes and they do diffrent things. Darken the inside with liver of sulfur or even a permanent marker, and that grey halo turns into a crisp dark outline that frames the picture instead of smudging into it. Lots of setters do this for translucent stones. Or go the other way and make the wall reflective, fine silver or gold, so it bounces light back up through the stone and brightens the whole scene. Fine silver on its own won't fix the grey though, since you're still seeing metal edge-on, it just changes the tone of it.

A gold bezel does read warmer, that's true, but it's a pricey way to solve a problem you can fix with a patina brush. If you've already got it seated, you might be able to lift the stone, darken or polish the wall, and reset without much trouble. One last thought on the floor showing through, some people deliberately back thin translucent stones with a piece of white or colored paper or foil behind the cab before final seating. Changes the whole mood of the picture depending on what you pick.

Did the marker trick on a chalcedony last year and it worked ok, but it rubbed off a bit when I cleaned up the excess. Liver of sulfur held way better for me. Also tried the foil backing thing on a thin piece and it honestly looked like foil, so test it on scrap first. Curious how you'd even lift the stone once it's seated though, does heat work on that or do you just pry carefully and pray?

On the lifting question, if there's no adhesive in there you just push the bezel wall back with a burnisher working in reverse order from how you closed it, points first, then lift the cab out with a bit of wax on a stick. Heat only enters the picture if somebody glued it, wich you realy shouldn't with a translucent stone anyway since any residue shows through like a fingerprint. One thing I'd add about the halo is that the dome height changes how much of the wall you see, so a low, flatter cab hides it better than a high one, which matters when picking your next teardrop.