Custom Object 2 & 3 Relationship with Activity object

Hi,

Does anyone can explain reason for each of the following cases/report column combination?

Report object combination: 

(Advanced Custom Object)

Opportunity, CO2, Account  >Result is not generated even if BoB for Account is set up (Role Step#2: Account-Can Read All Record "unchecked/No")

Opportunity, CO3, Account  >Result is not generated if BoB for Account is set up (Role Step#2: Account-Can Read All Record "unchecked/No")

CO2, Activity-Task/Appointment  >Result is generated, but only displays the very first Activity created within a single CO2 record (1 Task per CO2 record), other remaining Tasks associated to the same CO2 record are not displayed