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Just add a Clipping mask the the 'Landscape 1' image and... Voilà! The sky has gone away!
Reactivate all layers and put the 'Mountain' at the bottom, below everything.
To have a better blending we will add a Hue/Saturation Adjustment Layer only for the 'Landscape 1' Image.
We will set the Lightness to -44.
We don't want that this adjustment affects the whole image, but just the top.
Add a clipping mask and with the black color we will paint on it to cover the bottom part of the image.
We need to illuminate a bit the centre of the image.
Create a new layer above everything and with a soft brush and color white paint something like that:
Apply Gaussian Blur: 100px
Set the Layer to Overlay with Fill 35%.
Time to add some adjustments!
Gradient Map: Violet to Orange.
Set it to Soft Light with Opacity and Fill at 90%
Levels Adjustments to give a bit of contrast.