2. why was oxford, ohio, chosen by sncc as the orientation location? why did
sncc want to make it clear that the volunteers didn't "really have to go” if they
didn't want to?

Oxford, Ohio, wasn't supposed to be the orientation location. However, the Student Nonviolent Coordinating Committee (SNCC) had to choose a place at the last minute because the original place for the orientation, Berea College (Berea, Ky), backed out due to alumni pressure.  

Publicly the Berea College administration said they feared for the safety of the volunteers. Documents tell another story: they gave in to alumni pressure.

When SNCC emphasized the voluntary character of that action it was due to the real danger posed by what they were about to realize. SNCC feared for how far the South's racism would go due to past acts of violence.

this is a good question! i'll do some research and come back when i have the answer!