At between 5'10 and 11" I can almost certainly say a 58cm is too big.

FWIW, I'm between 5'8 and 5'9" and an actual inseam of 31.6" and I'm on a 52cm B2 with I think a 9cm stem. However, that doesn't tell the whole picture, because my arm pads are very forward with my elbows right on top of them. If I were to go with something like a Sytance aerobar that sat behind the basebar slightly, I'd need probably a 11cm stem. I could probabyl fit on a 54cm without any sort of weird or unusual setup.

I'm not surprised that they are offering you a 54 and a 56. To give you an idea the 54cm has 1cm less reach(horizontal distance from BB to headtube) and 1.5cm higher headtube. I've got a buddy who is about 6'0" and on a very aggressively setup 56cm.

I would say ask the shop with the bike in stock to try setting up the front end for you to try out. Put a stem on thats a little shorter and so forth. You're the one spending $2500ish you can be a little picky.

Good Luck