I agree with your assessment of these magnificent paintings. A study of them made in 1997 concluded that they were probably done in the 1820s by more than one artist and could have been based on a set of engravings that Mission San Gabriel had obtained from Mexico in 1771. They were actually painted by Indians from the nearby mission of San Fernando Rey.
Donald Francis Toomey, an expert on devotional art, in his excellent book The Spell of California's Spanish Colonial Missions says that the San Gabriel Stations of the Cross are painted using powdered mineral colors mixed with linseed oil and not paint made from flowers and berries, which is a story that is sometimes told. They are painted on unprimed linen canvas.
These paintings were exhibited at the 1893 ‘World's Columbus Exposition” in Chicago and later given to Mission San Gabriel by the Los Angeles Chamber of Commerce.
I talked with Donald Tooney and he said that Professor Norman Neurerburg published a book that contained half page images of each of the Stations of the Cross paintings. It is entitled The Indian Via Crucis from Mission San Fernando: An Historical Exposition.The photography was done by William B. Dewey. The book is out of print but available at many libraries and at most rare books shops (I found two copies were available at www.abebooks.com for under $20.00
Jackie wrote back and said: Thank you for that terrific response. I always heard the paintings were flowers and berry-based! They are truly unique and beautiful.