Would it be possible to start a second display server like we used to, one on ctrl-alt-F8 and one on ctrl-alt-F7? So when you want to do some gaming you start up your gaming workspace and switch to it, and when you want to turn off the nvidia graphics you destroy the gaming workspace, but your document workspace that was using the intel graphics is still there? That feels like it could be somewhat usable.
Should be. Windows is actually not crashing the desktop when the driver balks, so you can AFAIK run a Windows VM on a Linux and switch the PCIe GPU (yes, i915 counts from Haswell on) via KVM/QUEMU/IO-MMU between host and VM, with the latter not caring much, but the former crashing Xorg.
