unwanted automatic conversion to 32-bit when exporting/saving

Audition doesn't open MP3 files directly - it has to decode them. This is because it only operates using wav files, so MP3s have to be decoded first, and this is always to 32-bit files so that for instance, you can make amplitude changes to them without incurring massive bit depth losses. When you re-encode, the parameters that you specify for the MP3 are adhered to. So basically your observations are correct, but that's the reason for the behaviour. This behaviour was decided upon quite deliberately, in an attempt to minimise the damage done to MP3 files when opened by Audition. 

The damage happens, simply because there's a decode/re-encode process invoked every time you open and save an MP3, and that's unavoidable with lossy files. The losses to a file get greater each time you do it. Ultimately if you go through this process enough times, they'll end up sounding sort-of 'plastic-y', which is not good at all. There are one or two MP3-specific editors available, but what you can do with them is very limited, because of the way the format works. What it comes down to is that you can make rather crude cuts, and vary the volume a little, but that's about it without a full decode.