I ended up applying a difference blend mode to the marquee I drew around the stuff I wanted masked and that workaround achieved my goal. Not sure why the inverted mask didn't work but just applying a filter to the marquee itself within the layer mask will get the result for the Zoom video edit.
Hi, the mask works in an adjustment layer the same way it works on any other clip, it only modifies the content of the layer, not the clips below, so the mask won't "create a hole" in the clips below, it will just limit the viewable area of whatever effects you have applied to the adjustment layer. For example, if you make some Lumetri changes to the layer, let's say you increase the exposition, it increases the exposition in all the clips below, but when you apply a square mask to that layer, the exposition increase shown in the clips below will be bounded to the area limited by the adjustment layer's mask.
To apply a mask to several clips at once, I would nest all those clips and apply the mask to the nested clip. If nesting is not an option because of the sequence's distribution, I would create the mask on one clip, copy it, select all the clips I want to have the mask and choose the "paste attributes" option --- opacity to paste the same mask on them all at once.