It looks to me that the Reliant has a secondary hull in place of the Armstrong's middle third nacelle.
I don't think so. The Armstrong's center nacelle is larger than the other two, which accounts for the difference. It's actually the side-mounted ones that look funny to me, since they don't seem to have the unlit cap of most Kelvin-nacelles. And, of course, the bussards have been recolored to blue instead of red, but that would've been a quick change. Not that making the caps glow would've been much more work, especially if it was only going to be seen from a distance.
I figured if anything that Armstrong-esque ship would be the Stargazer NCC-2893, namedropped immediately afterwards over the PA while the Enterprise crew disembark
After I saw the movie the first time, I hoped that the Stargazer was actually the under construction soon-to-be Enterprise-A, but on further listening, the message is phrased in such a way that it has to be referring to an operational ship.