Followed these customizations to the letter, ran the singleplayer installer inside Wineskin, and it seems to be working like a charm. Running perfectly smoothly at 1680x1050, with no crashes so far.
For anyone looking to do this themselves on OSX Sierra, here's a little more detail that should help even if you're not familiar with Wineskin.
1. Download and install Wineskin Winery, and the Single Player installer provided on CnCNet
2. Open Winery and click the "+" under Installed Engines, choose WS9WineCX14.1.10 and install it
3. Make sure Wrapper Version says Wineskin-2.6.2
4. Select the WS9WineCX14.1.10 Engine and click Create New Blank Wrapper
5. You may get some questions about missing packages (Gecko/Mono) - Don't install them, click cancel.
6. Find the Wrapper you just created, ctrl-click and View Package Contents
7. Open Wineskin
8. Click Advanced
9. Click Tools, then Config Utility (winecfg)
10. Open the Libraries tab, type "ddraw" in the "New override for library:" field, and click Add - It'll ask you to confirm
11. Click Apply, then OK
12. Back in Wineskin Advanced, click Winetricks
13. Search for "dotnet20", check the box, and click Run; when it's done, click Close
14. Back in Wineskin Advanced, click Set Screen Options
15. Check "Use Mac Driver instead of X11", then click Done
16. Click "Install Software", "Choose Setup Executable", then choose the Tiberian Sun installer you downloaded earlier
17. Run through the installer - install all the components except for the Themes, uncheck things like Start Menu and Desktop. The movies are 1 GB and will take a while.
18. Back in Wineskin Advanced, click Configuration, then "Browse" next to Windows EXE
19. Select Westwood/SUN/Launcher.exe, then close Wineskin
20. Click on your Wineskin Wrapper like any other Application - it should open the Tiberian Sun Firestorm Launcher
21. Click Options, set your desired screen resolution (I use 1680x1050), select 'ddwrapper', check 'No Video Memory', click Apply then OK
That's it! Click on 'Singleplayer' in the Launcher to play the game.
On my machine I get a big white box covering up the Tiberian Sun logo in the Launcher, but that's the only issue I've come across so far.