XPS 15 9570, not able to Install Windows 7, NVMe PCI-E SSD

I'll answer your question but then explain why it won't matter.  First, you can't use Windows 10 NVMe drivers on Windows 7.  There are hotfixes to add NVMe support to Windows 7 that you can inject into both the Boot.wim and Install.wim files on your setup media, but that's a bit of a pain and injecting those hotfixes seems to break the install media when used on other systems.  The much easier solution would be to just switch your system from AHCI to RAID mode, since that activates the Intel Rapid Storage controller and abstracts the NVMe interface from the OS.  At that point, you'd just need to supply the Intel Rapid Storage driver (get the "F6 floppy" version from Intel directly) at the point where you choose which disk to install to.  Just click Load Driver and browse to the folder containing the extracted contents of the ZIP file you downloaded from Intel -- no need to inject anything or deal with hotfixes.  That's the configuration that Dell used for systems they shipped that had Windows 7 running on NVMe SSDs.  (NOTE: If you currently have a Windows 10 installation that was performed while the system was in AHCI mode, you'll need to change this setting back to AHCI to get that installation to boot again.)

Now, the reason none of the above matters in your case is because your system won't work with Windows 7 at all anyway.  Microsoft only supports Windows 7 on Intel Core CPUs up to 6th Gen, and you have 8th Gen.  Since Microsoft doesn't support Windows 7 on anything newer, Dell doesn't provide Windows 7 drivers for these systems, so even if Windows 7 worked properly (which isn't a guarantee at all), you'll probably find that other parts of your system don't work properly or at all.  And even if you somehow addressed all of that, Microsoft will stop providing security updates for Windows 7 at all in just over a year (January 2020), and if XP's retirement is any indication, browser vendors will stop providing updates on Windows 7 shortly thereafter, at which point that system will arguably become unsafe to use on the Internet -- and eventually lack of browser updates will probably create functional issues when browsing.  If you've ever tried to get a new XP SP3 install on the Internet recently, it's quite a challenge because it simply doesn't support multiple technologies that are now required on the modern Internet.