Like you, this first inspiration photo is my favorite and I think we could definitely duplicate this look for your stairway. I'd love to use a mix of gold and white frames and some family photos as well as bright and colorful artwork and prints.
I love the idea of this more lined up/matching type of arrangement for the bottom of the stairs/hallway where you discussed wanting some additional items. We could do a cluster of family photos for that area (either in color or all black and white).
Below are some of my picks for the gallery wall to get us started. This gold Target frame (below to the right) is a great option that is low-profile too (would also work for the hallway area) and it comes in a variety of sizes. A lot of these bright prints/artwork I have included already have frames which is a plus.
As far as your built-in shelves I really like all the items you have so far and the idea of wallpapering the back of the shelves. I would love to add some colorful old books to stack and fill the space, as well as some more bright + colorful accessories. We can also choose something coordinating for the mantle on each side of the television.