Thanks Hans for clearing up the module's behavior when 'out of circuit.' As I alluded to in the other thread I mistakenly created, the module is currently working and my U3S has been running overnight transmitting WSPR and QRSS on 40 meters. The repair probably boiled down to inspecting it with a 'fine-toothed comb' as well as reheating connections and connectors. Probing the Si5351A chip is a challenge and I found a small sewing needle to be a great help.
I used a 'proto' type of breadboard to mount and power the Si5351A module to make it easier to troubleshoot away from the U3S. Not that I could get a lot of testing done with it, but it's got me thinking about doing some experimenting with the I2C bus structure. Maybe an Arduino sketch to simulate some commands...or just build some jumper header extenders.
But, away from the 'mother ship' the little module stood up pretty well; "took a licking and kept on ticking" as John Cameron Swayze used to say about Timex watches.