The Dangers of Microsoft Pluton

Can RISC-V save us here? Or is it time to start hoarding CPUs?

It's time to start rejecting such corporations. Nothing else would work.

I would assume chinese made RISC-V have their own special sauce.

That's a big assumption.

...if the schematics and tapeouts are entirely public.

Otherwise you can be assured that there will be backdoors.

Might be a blessing in disguise?

The libre computing movement got lazy. We got used to care about free software and just accept free-riding on non-free hardware because "hardware too hard" and frankly we got it easy with x86 CPU and PC manufacturers being generally friendly, actively or passively, to free software and actually benefiting from industry concentration. The less attractive proprietary CPUs and other chips get, the greater chance a small but lively open ecosystem develops?

You can post hoc modify circuits so they look like doing logic A but they actually do logic B by adding new p or n junctions.

In theory, yes. In practice it is not realistic to implement a plausible-deniable hardware backdoor targeting all CPUs being manufactured while keeping the schematics and tapeout open.

While the same CPUs are even fabbed in different locations around the world.

While also going undetected for years and while none of the engineers involved blows the whistle.

In short no, you can get away with a targeted attack but nothing so massive.