Surprised you got no responses.
I can't remember whether the bumps line up with the flats or not. I may have some old bushings kicking around to check.
As you suspected, if they are all facing the same direction, they are not installed correctly.
As you stated, the front bushings should have the flats facing horizontally, and the rear bushings should have their flats facing vertically.
How significant is this distinction in terms of handling?
I'm really not sure, but I would like to hear from Steve!
This orientation would seem to encourage a slight toe-out adjustment as rolling resistance increases.
Of course, it may only be relevant under heavier turning loads, in which case it seems that this would encourage mild understeer, or at least a slightly delayed reaction to steering input.
I'll even float a 3rd theory (why not, I'm on a roll!): The spring exerts more downward force on the front bushing than the rear (due to its position), so this orientation simply 'levels' the control arm.
Either way, I would speculate that this is not the cause of your vibration.
Not sure if the 'squeezing' you mention is an indicator of the condition of that bushing, but I will note that the 'top' of the bushing is under load while stationary, because the spring is pushing it downward.
1986 300E 5-Speed 240k mi.