I like the Krylon brand and use the UV Archival Varnish spray on my oil paintings. I have had to remove it on an oil painting to sign it and had no trouble doing that. I have used it on watercolor/colored pencil mixed media and watercolor. I always test first. I did have one watercolor that had subtle passages and it lost some after spraying the test piece. I always have a watercolor scrap on the side of each I do to color check before laying down a wash. It matches the same colors and transparencies of the painting so I’ll use it to test spray varnish first.
You do have a situation where she is using delicate mediums too so whatever you try always test test test first. Test on each medium separately and together if mixed media. Don’t just spray finished art work
https://www.krylon.com/products/?N=159827624+0+198512443