When you draw a freebody diagram of a mechanism please name the joints and number the bodies. It makes it easier to communicate (most advances in techology and engineering are because of increased ability to communicate).
Do this: Make a joint chain D-C-E-F make a inverse kinematic chain foem D to F make a joint chain A-B make a inverse kinematics siolver A-B parent the effelector of A-B to joint C-E. This should work as long as you dont have a lock up. Note this is as good as it gets without going intod more complex setups where you manually solve the mechanism.
PS: It will never lock up as long as 1 and 2 are equally long and paralel. Or you only have only one fixed point (A or D) and make a bar between A-D in which case its a standard industrial robot setup.
PPS: Yes I could do a fancier solve too, I'm a mechanical engineer after all. But then I would have to send you a bill.
