Have you checked out all the sources? I stepped through this lineage, and each step has numerous sources, some are well respected, such as Richardson’s numerous works on medieval genealogy, but others seem pretty cryptic and are worth verifying. It seems the information to answer your query may be here, but some of the notes suggest work is needed.
My suggestion would be to download this family tree as a gedcom, and generate a report that attaches all the sources to the relevant facts, and reviews the data at each step of the way.
I would do this only for the line of descent (ie, a “register” report) from Edward I to Agnes Kemp.
If you decide to do this be prepared for a lot of work just to format the report, and this is before reviewing any of the sources. Reviewing the sources would include reviewing the sources in the cited references (in selected cases) as well.