reesepbuttercup - Nice picture! You can't spot clean, but the Gumbacher's or Winsor & Newton cleaning fluid is inexpensive and easy to use. Any art supply store would have it. A 14x16 frame is actually a common enough size. Try the various hobby and frame shops when they are having a sale. Or this web site. That would be great in a weathered wood frame. https://www.arttoframe.com/14x16_picture_frames  Gracie - You are kidding, right? Perhaps a brass brush could be used on the stubborn spots?

One way to make a particular piece of art stand out is size. In the mockup below, the painting is the largest of the group. Another way is frames. All have black frames except for the painting. Another is color. the painting is multicolored. All the other items in the grouping all have muted colors. So they essentially look like framed black or white piece of paper. I don't think any of the pieces that you have posted would work because the only possibility is the black and white flower but the black is so bold against the white paper, it stands out too much. Also, I see no problem mixing different media in a gallery wall including poster, prints, photos, original art, etc. Here is an example of three black and white photos along with an original painting.

who is the artist

You will not like my reply but 30 years ago I was given a Victorian oil painted screen and I cleaned it with nail polish remover. Acetone. It took all the yellow off beautifully. 30 years later. They are still hanging in my bathroom in bedroom and are beautiful. Maybe try it?