A couple of questions:
1. I have ordered from GRO in the past. Is this where I will order the birth, death, and marriage records you have referenced?
I order from the GRO -https://www.gro.gov.uk/gro/content/certificates/Login.asp

2. Can you direct me to the proper location to order parish records, if that's possible? The transcriptiopns are on this site - if its images that you want then best ask Graham Davidson

3. I'm wondering, where did you find a reference to William's death being inscribed on Sarah's headstone in the Cuba Cemetery in Illinois?
Buried in Ancestry !