1. Only safe to assume if it says (2 x 8GB) between brackets. Otherwise it might just be 16 GB single stick.

2. 4 slots in total, 2 white slots and 2 black. 2 white slots is where the dual sticks go, all 4 slots is where quad sticks go.

3. Because that combination has not been validated by Dell. Only the validated configurations are listed and supported.

The "help me choose" help section for the memory ordering page will not help you much, as it will take you to a help page that has nothing to do with AMD memory configurations: https://www.dell.com/en-ca/shop/help-me-choose/cp/hmc-memory-consumer

None of that applies to AMD systems as they do not support Intel Optane. With an R10 none of the listed configurations in that table are possible. 

When it comes to AMD systems like the R10, you just have to guess what Dell recommends, since they will not tell you.