The ETT you're looking at (56.5 to 57cm) is what I use as well (with a 110mm stem) . But I'm 6' with a 35.5" cycling inseam, and my knees hit my elbows so it's pretty short for me. It sounds like that would be really short for you.
The problem with going to a smaller frame is seat to bar drop. Smaller frames have a shorter head tube. So you will need more spacers and/or a flipped up stem to get it to fit. It if fits at all. I have a significant (10cm or so) drop on my R3SL and that's with 20mm of spacer and a turned-up stem. I like the look of the turned-up stem and it makes for a more direct connection between fork and bars, but some people don't like it on their bikes because it does not look "pro".