Reaper does not downsample to play back at 24bit. 64bit floating point is not a conversion. Read up on this, you are somewhat correct about some of your info but not that. I unfortunately am out the door right now and have no time to delve into this, but I felt compelled to add some input.
Here's a start though:
http://en.wikipedia.org/wiki/Audio_bit_depth
Also, sample rate conversion has nothing to do with dithering. Dithering is when you are changing bit depth. Reaper actually does a great job at both, I have done some extensive testing with this and have recently ditched R8Brain and Sox for sample rate conversion (the files nulled to a ridiculous low level). But Dithering is a completely different thing with completely different requirements and times it's needed. You have some awesome and accurate info in your great post, but you are also giving out some misleading info.