Sorry, but I was a bit premature with my questions.
"Could my mysterious Isidora Sise be your Isidora Saiz/Sais?"
"Have you any record or lore of what became of Ambrosio Madrid and Isidora?"
Answer to the first is "No" - and answer to second is "Yes" in that with a little effort I found both Ambosia and Isidora doing quite well as husband and wife after the turn of the century.Even old Jose Felipe Madrid was still around in 1900 (age 90).
I created a pretty complete census transcription collection of this Sais and Madrid line from 1850 to 1930.If anybody can use it, just contact me and I'll email it to you.