Photoshop :: Unwanted Color Gradient After Photomerge

I got an annoying problem with a weird color gradient that appears where it shouldn't. My context is, that I'm producing underwater photomosaics. I use Photomerge for merging, which works great for the often uniform seabed sediment images.
 
[merge1.jpg]
 
The example is 65 images, btw. After placing the images manually within Photomerge, all looks great and ready for a perfect result after rendering (see above). However, when doing so, the final merge from within Photomerge (by clicking the OK button) creates perfect smooth blending, but a weird color gradient crossing the whole mosaic.

this might originate in the fact, that the individual images also are not perfect, due to the uneven lighting situation underwater. In case all images are a bit reddish at the top and yellowish at the bottom, the final gradient over the full mosaic shows that as well. So to me it looks as if Photoshop just tries to be too smart and thinks, reapplying that pattern to the whole would be a great thing to do, which it isn't of course.
 
[merge2.jpg]
 
I discussed this "bug" with Adobe staff and they recommended, to create the merge with the "Blend Images Together" option in the "Load" dialog unchecked. This creates a mult-layer document after rendering, with the images being placed properly but not blended.
 
[merge3.jpg]
 
From here on, Adobe staff  recommended, I shall use the "Auto Blend Layers" function (File menu), but the result is identical as image 2. Ok, then, they recommended, to uncheck "Seemless Tones and Colors" in the "Auto Blend Layers" dialog. Indeed, this gets rid of the gradient, ...
 
[merge4.jpg]
 
... but shows very hard edges of the images, which get very prominent after optimizing levels:
 
[merge5.jpg]
 
Was that understandable? How this could be handled in a way, that my resulting mosaic looks as perfect with respect to the original colors as in the initial merge window before rendering it (image 1)? Maybe it's a specific Color Setting (Edit menu)?
 
The problem appears, btw, in CS3 to CS6!