> Going to Venus would allow us to simulate a mars free return trip, but in only 1 year (which would mean for a mars-class ship, a minimum safety factor of 3 (really, six) for fuel, water, air, food, medical supplies etc), instead of a 3 year mission with only a safety factor of 2.
Simulate what ? Sitting in place ? We can do that on earth orbit. Engines running for long time ? No need for people aboard for that.