OK on your further tests:
Wasn't sure from your posting whether your test mapped drive T: worked for both photos and videos, or only for photos. As network access is working for photos, that suggests that the basics of Windows networking is OK, and the problem could be something to do with how the video files are stored (any strange file/folder permissions?), or a problem in Studio...
DirectX 12 is a standard part of Windows 10 - you won't get rid of that in a hurry. But Studio probably isn't using it (for backwards compatibility with Windows 7/8/8.1 it would need to be using something older (9/10/11). But a DirectX error would IMO be more likely to cause instability, problems with drawing user interface elements, problems previewing etc. Display driver issues would IMO be more likely to cause instability, UI drawing, preview and export problems, not a networking issue...
Studio iteslf and the Studio background processes shouldn't need any special handlng in the Windows firewall over & above anything done by Studio's installer; network access is needed to the local machine only (a throwback to the days of Avid Liquid, that did support editing and exporting in a networked environment, with multiple workstations).
My "gut feeling" is that there could be something strange about the NAS folders in which the video files are stored - especially if you can't even see those folders by clicking the '+' beside the drvie's entry in the tree.
- Were those folders created manually by you, or were created by some app you were using to copy the files from the camera onto the PC?
- If you manually create a new, empty folder on the NAS, can Studio's Importer see it?
- If Studio's Importer can see a manually-created folder, what happens if you use Windows Explorter/File Explorer to copy various "importable" assets (photos, video files, audio files etc.) into that folder? When you go into the Importer and browse that folder, can Studio now see what is inside?
If Studio's Importer seems to work OK with the manually-created folder, carefully check the ownershiip and security settings for the files/folders that the Importer cannot see, in the hope that you can spot any significant differences...
HTH
Richard