A lot of per-application schemas on NixOS show the No schema found note. I think this is because of how Nix handles applications files, and I’ve rarely run into any problems this approach causes (though this might be an exception).
I did some experimentation around the default-viewmode for Files, and can reproduce your issue. It looks like the reason for this is that setting default-viewmode isn’t sufficient in and of itself; tab-info-list also needs to be set (you can see this second key change after switching the view mode in Files and then closing the window). Digging around in Files… well… files reveals that tab-info-list is a tuple with the following schema:
<key type="a(uss)" name="tab-info-list">
<summary>Details of open tabs</summary>
<default>[(0,'','')]</default>
<description>Array of tab info: View mode, root uri, tip uri (for Miller view)</description>
</key>
The “view mode” here corresponds to 0 for the icon view, 1 for the list view, and 2 for miller_columns. The two strings are of the form file:///path/to/directory, with the second string being empty for the icon and list views.
Unfortunately, home-manager can’t yet set dconf tuples (though there’s a pull request open to add this functionality), so it doesn’t look like File’s view mode can be fully set yet. 