I just put an anchor on the button and I put like you said but there is a problem:
Scale inner:
iOS: img.imagesia.com/fichiers/hf/ios_imagesia-com_hfmj.png
Android: img.imagesia.com/fichiers/hf/samsung_imagesia-com_hfmk.png
Scale outer:
iOS: puu.sh/7BWNZ.jpg
Android: puu.sh/7BWw2.jpg
In Construct 2 the button is located at the bottom right
And the background is always cut at the bottom on android
Parralax: 0,0
Scale rate: 0
Button > Anchor
Left edge: Window left
Top edge: Window bottom
Right edge: none
Bottom edge: none