The problem in your original layers is that both photos are a bit away from each other. And masked area is a bit narrow. That's why your mask isn't working the way you want. It can still work if you increase the size of your brush to mask. But I guess it would cover significant area of your overall composition, which you might not want.
So masking isn't a ...
As others have said, in Photoshop it's always best to keep your images on separate layers, then you can move them independently. There's lots of ways you could fix your problem. Anyway, here's one take on it.
You can group the image layers, then apply the layer mask to the group.
Then, you can select the layer mask, unlock it, and move it, or even stretch it ...
In order to avoid the stripes, I suggest the following:
Create the map in really high resolution 4000x4000 or even more.
Apply a little gausian blur when it's done to smooth it more.
In brush settings adjust spacing to minimum.
Tip: You can try the Liquify filter to create these flares, although it's not exactly the desired result but it might be useful.
A simple way of making sure you only have a certain number of colors in a gradient is to create the gradient in a document that only has as many pixels as you want colors.
So if you for example want 23 colors, you create a document with the dimensions 23 × 1 px.
Then you select the colors you want as foreground and background color and add a linear Gradient ...
There are several ways, but extrapolating from the method you appear to be using; central black stripe with vignetted mask, I'd do it this way.
Split your two images to separate layers.
Add a Layer Mask to each.
On each, paint your vignette starting from the exact edge of the image to the distance you wish to fade. It's this exact edge that gives you your ...
I think the problem is the posterize filter.
Instead, try converting to Indexed colour mode, set 50 colours, then convert back to RGB mode. Then export as PNG.
I tested this method, and checked the output using Image Magick, and it definitely shows only 50 colours.
If you've installed Image Magick the command to check the number of colours is as follows: