Read the Theory Manual of how the Inertia Relief calculation is done to answer the last question.
Here is the first sentence from that page:
An equivalent free-body analysis is performed if a static analysis (ANTYPE,STATIC) or a buckling analysis (ANTYPE,BUCKLE) is used together with aninertia relief(IRLF,1). This is a technique in which the applied forces and torques are balanced by inertial forces induced by an acceleration field.
Inertia Relief models can have no supports.
Why would support location change the value of accelerations? Because the stiffness matrix changes when the support location changes.
How should I know about the best location to apply supports? Inertia Relief is problem specific. Show the geometry and the support choices you are considering along with the applied forces and moments for anyone to answer a specific case.