Upon boot, am presented with blank screen

I recently helped my brother upgrade his monitor setup and video card from an ASUS 750Ti and single Dell 3007WFP to the MSI RX 470 and two additional Dell SE198WFP on each side. (The 30" had its backlight die, another story for another time, so in the meantime, I've got a Samsung 2253LW in the menatime. I am still using the original dual-link DVI cable that came with the Dell monitor.) To hook up the side monitors, we used some HDMI-DVI cables. (I need to check if the cables are single-link or dual-link.) Installation and orientation setup was successful in Windows 10, but in Ubuntu, all that happens is it appears on the side monitors, duplicated, in 1024*768.

Originally, I thought it was the open-source drivers not communicating with the card properly, so I installed the AMDGPU-PRO drivers off of ATI's Web site. Unfortunately, not only did it not resolve the incorrect resolution/incorrect physical orientation issue, but now Unity is not loading correctly. I am able to right-click on a black desktop and open applications (such as a browser, which I'm doing right now), but I have no window decorations, and I am unable to move or resize the window.

Display settings under unity-system-settings only says that "internal display" is connected (laughable), doesn't say anything about the Samsung monitor, and provides no option to not mirror the Dell monitors.

Xrandr: xrandr: Failed to get size of gamma for output default Screen 0: minimum 1024 x 768, current 1024 x 768, maximum 1024 x 768 default connected primary 1024x768+0+0 0mm x 0mm 1024x768 76.00* Xorg.0.log: http://pastebin.com/aytvJzVc

dmesg: http://pastebin.com/Akp2qQyP

lightdm.log: http://pastebin.com/RnEa6DEc

Recently, I tried the suggestion at http://askubuntu.com/questions/17381/unity-doesnt-load-no-launcher-no-dash-appears, and even reverted to the open-source driver, but that didn't resolve the issue.