HMmm……
As far as blending modes go,…what version of AE? 6.5 will let you do it fine. In an earlier version,..set all your blending modes in the pre-comp. THEN when you collapse they will pass on thru and be correct in the main comp.
As for the camera,…..the camera in your MAIN comp is what will now affect things. If you have a camera in your pre-comp it will create weirdness once collapsed into your main comp. If you are trying to retro-fit your scene with these techniques you may need to re-do your camera. “-( Also,…once you collapse, your x,y,z of your pre-comp elements now become ‘relevant’ as an offset to the precomps x,y,z placement in your main comp, check your Z placement of the precomp as you are now dealing with the z position of elements in your pre-comp added to your z placement of the pre-comp within the main comp (e.g. element in z comp at z of -100 AND pre-comp placed in main comp at z of -200, then that ‘element’ will be at -300)…..