Sequencer retimer

I’m wanting to retime specifc animations in the sequencer, would welcome any thoughts as to how this could be achieved. What I’m wanting to do is have animations play back at 12fps in the sequencer but keep the camera (and some other effects and things) at 24fps. I’ve tried keying the animations to be at 12fps with stepped keys but I lose simulations and other interactions. I figured the best way (and this is what I do in 3Ds Max if needed) is to render every 2nd frame - ‘even’ numbered frames, then shift the animations backwards by one frame (ignoring camera keys and any other keys I want to keep at 24fps) then I render the in between ‘odd’ numbered frames. I thought I might be able to play with the play speed in the sequencer as it would keep motion blur correct and so on, but it will also apply to the camera, and if I set it to 0 to hang on a frame it will not continue.
Any thoughts as to how I could automate this would be brilliant! Thanks all