Got me a nice surprise!
When I re-enabled the pre-Fermi iGPU (NVidia ION1, a 9400GT equivalent), it was recognized properly and WinX automatically installed driver v341.74. Fun part is, I can't even find 341.74 on NVidia website. Closest thing is 341.44 WHQL which is the last driver to support pre-Fermi GPUs, so I'm
guessing NV/MS did make/keep a legacy driver for WinX automatic updates after all. Another guess of mine is that 341.74 supports the same GPUs as 341.44. My GT430 certainly was old enough to be supported. Alas, the only Maxwell GPU supported by 341.44 is 750Ti.
Bottom line: now I'm able to use the iPU for display (plus some BOINCing) while the GT430 is fully dedicated to folding once again.
Idk what went wrong during my WinX upgrade, but looks like the same GPU support that WinX Tech Preview had is still in place in the official release. If you want to mix pre-Fermi and newer, I have a hunch that it'd be a good idea to temporarily remove all but the pre-Fermi GPUs during upgrade. That should force WinX to use 341.74. Put newer GPU(s) back after the upgrade.
I don't think it very likely that there will ever be another driver update for legacy NV GPUs, and since this is coming from WinX Automatic Updates, it's 341.74 or bust with legacy NV GPUs and WinX. So no CUDA 7.x, this driver has CUDA v6.5.