1)
Why did they test them with the word Shiboles?
1.
Radak #1: Really, they would test them with any word with a Shin; the verse mentions one example.
2.
Radak #2: They tested with Shiboles due to Shiboles ha'Nahar (the river current). Perhaps the air caused that they cannot pronounce Shin, just like people of France do not know how to say Shin, and say it like Sav.
