Photomerge mask issues

[Mollusk222]

I'm currently stitching together images of architectural drawings and in a world of hurt. I am using photomerge in Photoshop CC and the auto function is surprisingly good for most of them. It's so good in fact that I only need to make a few adjustments to the masks here and there to get things to line up properly.

The one catch here is that photomerge is doing some sneaky business to equalize the exposure (I did set up copy lights and tested for even exposure but there are still slight nuances across the frame that photomerge miraculously blends) All this to say that when I go to make adjustments to the mask which is a pure black and white mask created by photomerge the adjustments I make at 100% immediately reveal exposure issues. So the mask is without any blending properties as far as feathering or grayness is concerned but when I change it using the same pure black and white selection style it immediately reveals a completely uneven exposure value across the entire new selection I made on that layer.

Is there a way to adjust the mask without losing the lovely evenness that photomerge created? It's a real mystery to me.

These images show a before and after for a simple 100% black and white adjustment to the layer mask. Instead of intersecting into the drawing I would prefer to just cleanly join them in the empty space between. I have several techniques for blending and resolving the issue but would prefer to make an adjustment to the mask that keeps the original blend alive instead of performing several extra steps. Help me! The adjustment was on the middle layer in this picture.

[Mollusk222]

Looks like I stumped y'all....

[Norman Sanders]

If you are willing to set aside mask retouching in order to equalize background tone and, instead, use Photoshop's Blend If command, with a simple slide adjustment, it would provide the option of elimintimng the current background and replacing it with a flat color and tone level of your chooseing.  (Click on the double arrow in upper tight corner of the illustration to see a larger view.)