Hey Nate -
The descriptions are sometimes tough to follow. The 'rosette' reference has to do with what can bee seen via a tight, directly top-down veiw of the A. tenuis branch. None of the shots above really shows off the rosette formation of the coralites, though your first shot comes closest.
Personally, when trying to i.d. Acropora I first look at individual coralite formation and spacing along the branch. It's still tough to tell with a frag though. They can just look so different from a mature colony, depending upon where on the parent colony they were clipped from. I'm still never sure whether I'm looking at A. humilis or A. samoensis, as frags or small colonies for example. They are close, and frags seem nearly identical (to me). But large colonies are easy to distinguish from each other (again... for me).
I have spent hours in front of my tank and the tanks of other folks staring at Verons books... just trying to get certain basics of certain corals into my head so that oddities really pop out when I see them.
I have what I beleive to be A. sarmentosa and A. samoensis in my tank. Sometimes knowing what something 'isn't' is easier than saying what it 'is.' (Even though it can seem aggravating sometimes when someone does that!)
I realize that skeletal examination is the only true way of i.d.ing a coral... and even then it can be sketchy (see Borneman's thread on coral i.d. on RC)... but I don't care. I like the challenge, and I think it's really educational to delve into the books to give it a try.. for sps, lps, whatever.
Thanks for asking.
b