The next step is to simply abandon Manipulate altogether and build your interactive with DynamicModule. Ultimately that is what I always end up doing if I start by creating a Manipulate. Typically though I almost always start with is, in fact, a DynamicModule: it gives you complete detailed control of your design.

The secret trick to obtain complete control of locations of controls is to use Manipulate[None,....] and then simply use Grid to layout everything. The dynamic content itself, can now be placed anywhere, and the controls in any location in the main display area. I like to use this method since now the controls can be placed near the dynamics it is supposed to manipulate. I found this method sometime ago by accident when I was trying to figure how solve similar problem you are having. With this method, any possible layout can be made very easily. Forget about ControlPlacement, and use the powerful Grid :). ps. Also Item can be very handy here.

Manipulate[None,
 Grid[{{Grid[{{Control[{a, 1, 10}]},{"Caption of b"},{Control[{b, 1, 10}]}}],
    Grid[{{Control[{c, 1, 10}]},{Dynamic@Plot[Sin[a x] + b c, {x, -2 Pi, 2 Pi}, ImagePadding -> 10, 
         AxesOrigin -> {0, 0}, Frame -> True]}}]}}, TrackedSymbols :> {a, b, c}]

Here is a demo that uses the above method DynamicAnalysisOfASecondOrderSystemWithHarmonicLoading