In hindsight, this was a pretty simple fix... But I didn't see it documented anywhere.
Because my system was 64-bit, the registry fix, as mentioned in my original post, needed to be applied under the Wow6432Node.
(Registry keys specific to 32-bit applications are stored in a branch under HKEY_LOCAL_MACHINE\Software\WOW6432Node)
I believe KB974537 “FEATURE FORCE POPUPS ONTO MONITOR”, should be updated to include this information for 64bit users.
Recap of the fix:
1. Open registry editor – regedit
Start > type regedit in the search box > hit enter
2. Browse to HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Internet Explorer\Main\FeatureControl
3. On the Edit menu, point to New, and then click Key.
4. Type the following key name, and then press ENTER:
FEATURE_FORCE_POPUPS_ONTO_MONITOR_KB974537
5. On the Edit menu, point to New, and then click DWORD Value.
6. Type iexplore.exe, and then press ENTER.
7. Right-click iexplore.exe, and then click Modify.
8. In the Value data box, type 1, and then click OK