So I have an answer, granted, it's not going to be an answer that anyone likes, but at least it's an answer.
I spoke with my Senior Advisor today, and he informed me that the Mac will only support resolutions up to 4096 for an SST monitor. In order to get the full 5120 resolution, the monitor needs to be MST. Unfortunately I have no idea what the difference between these two configurations are, other than a quick Google search. I even tried to do a search on SST & U4919DW and didn't get any hits, which leads me to believe that monitor manufacturers don't post that information. If anyone has some good articles on the difference, I'd love for you to post them so we can learn more.
It still leaves me wondering why the monitor is only getting up to 3840 instead of 4096. My Advisor is scheduled to call me back next week with an answer.
I also reminded him that several individuals on this forum were able to get full resolution running Windows on their Mac, which means that the SST/MST limitation is a software limitation rather than a hardware limitation. And if running Windows on your Mac solves the problem, then certainly an OS update could solve the problem as well. However, since the SST/MST problem is a known limitation (intentional?), I doubt that Apple would remove this limitation. But that's just a guess.
It would seem that the next logical step would be to return the monitor to Dell, and then to investigate on whether the other three models that are being released (499P9H, 49WL95C, CRG9) are MST monitors. If they are not, then there would not be much hope of getting them to be compatible with the Mac.
He also mentioned that I could go to Apple.com/feedback and suggest that they make the Mac compatible with an SST monitor. Granted, I'm sure that with all the requests they get, this one won't be too high on their list, but I figure that it's worth a shot. Perhaps if enough people point out the same thing, they would be interested in moving on it.
Thanks to all who have contributed to this forum, it's been helpful to me, and hopefully we can all find a solution that works for us.