would this product work without having to purchase the pan tilt and 3 axis gimbal?

Yes, the Proaim 18ft Jib can also work perfectly without the Pan Tilt Head and 3-axis Head. You can accommodate small DSLR/DSLM cameras directly on the camera mount top plate of the Proaim18ft Jib.

Could it be controlled by a computer

The Proaim Pan Tilt Head PT-SR is only controlled by a joystick controller included in it. It will not be controlled by a computer.

Can the Sr. Pan Tilt Head be moved by hand if i don\'t want to use the motor?

Yes, the Proaim Senior Pan tilt head can be moved with hands but before this you have to disengage the motor gear to avoid any damage to the gears.