When soft proofing, uncheck preserve RGB. Then tick "black point compensation" and leave "simulate paper type" & "simulate black ink" turned off. Choose perceptual to keep the shadow details in, or relative colournmetric to keep the exact colours and tones but you'll loose more shadow detail.
First bit correct enough, second bit not so. The rendering intents are about remapping the entire gamut of colours to a new gamut, not specifically about shadow detail. The only two intents you need worry about are Perceptual and Relative Colorimetric and the difference is thus,
Perceptual will remap ALL the colours to the new gamut keeping roughly the same percentage difference between tones. So say you have an out of gamut red and an in-gamut red, it will shift BOTH inward (if moving to a smaller gamut) until the out of gamut red is in-gamut - but it will shift the original in-gamut red further in to keep the same difference. The clue is in the name, it will move all colours, but should remain perceptibly similar.
Relative Colorimetric will only remap the out of gamut colours. Anything that is in gamut will remain exactly as is. Sometimes pictures only have a few rogue out of gamut colours which just need clipping in to fit the print gamut, in which case it's generally a good way of maintaining colour accuracy across the majority of the image with just the few out of gamut areas reduced to the limit. It doesn't work so well if lots is far out of gamut, because then it's clipping larger areas down to the gamut limit - This can result in nice graduations that were all out of gamut just being crushed to one tone.
If in doubt I go with Perceptual
You have the right idea in having a file duplicate tiled next to your original with the dupe having the softproof enabled. Then you can adjust the image to best match the original RGB file within the new gamut.
Oh and you can use the 'Simulate Paper Color' and 'Simulate black ink' option providing you have NO other on-screen elements (Checking the first will result in the latter automatically being selected and greyed out - you should have both on if you're doing this). This tries to simulate the paper contrast ratio on your monitor (paper very low, monitor much higher) so will look rubbish if there are other high contrast elements (panels, tabs, etc) to compare with. Cycle F to get your full black screen view and hit Tab to remove the toolbars. Close your eyes and hit CTRL + Y to enable the softproof and adjust to the new image. You obviously can't do any adjustments when this is enabled, however.
Of course this is all largely pointless if your monitor is still too bright or your print viewing conditions are too dark. So sorting your screen/ambient luminance first will likely result in much better screen to print matches before anything else
