Could the Roberts Commission see Magic intelligence?

Would the Roberts Commission have known that intelligence was not supplied to Kimmel?

No; the investigators were not read into Magic and could not examine intelligence whose existence remained secret from them.

The Roberts Commission was not told that the intercept-derived intelligence had failed to reach Kimmel. Its members were not read into the existence of the highly secret codebreaking material. They therefore could not reconstruct who possessed the intercepts, who received assessments, or where distribution failed. Wartime secrecy protected the cryptographic capability while also narrowing the commission's account of responsibility.