Switched back to AHCI mode.  Installed the Intel SSD, left the NVMe drive disconnected.  Reproduced the problem exactly as described earlier, but I caught the drive disappearing.

Boot to Win10 1703 installer, windows sees the SSD.

I SHIFT+F10 to open the command prompt, run diskpart to show the drive, make sure it's reporting correctly.  It appears to do so. Then I issue a "rescan" command and relist the drive.  It has disappeared.

I refresh the windows installer drive selection window, and the drive is now gone.  I brought the command prompt back in front.