Soo sorry to hear about your uncle. {{{{HUGS}}}}
1. What can I put behind each siggy block to stabilize it for persons to sign it after its been quilted and that will survive several washes?
The siggie block itself shouldn't have to be stabilized. The one thing I would recommend are these pens by ZIG (in the scrapbooking section at JoAnn's). They have a ballpoint on them and are really easy to write with no pulling. Pigmas for me, because of their fine point, catch on the threads of the fabric and aren't easy for me to write with at all. Also, remember to heat set the signatures before washing.