What would happen to the concentrations of Pyruvate, NADH and intermembrane H+ if the ETC stopped working?

"Proton decreases" would be the right approach.

Explanation:

The Kreb cycle or system plays a key role throughout a matrix of mitochondria. 3 NADH, 2 FADH2 as well as 1 ATP are synthesized. Whenever the Kreb process has been halted, the acetyl CoA throughout the Kreb cycle has not been used. Pyruvate doesn't at all, disintegrate via acetyl CoA. Therefore in cells, pyruvate accumulates. Unless the Kreb loop is halted, therefore the concentration of NADH and FADH2 declines.

Thus, they aren't used for the transportation chain of electrons. This is also why protons have not been discovered to be injected towards intermembrane space.