Has any US SC case involving the 2nd in the last 150 years ever cited the 14th Amendment as validating the RKBA at the state level?
No, but a number of federal lower court rulings have cited that the RKBA is not validated at the state level.
"The second amendment declares that it shall not be infringed; but this, as has been seen, means no more than that it shall not be infringed by Congress. This is one of the amendments that has no other effect than to restrict the powers of the national government ..."
-- United States v. Cruikshank, 92 U.S. 542, 551 (1876)
"But a conclusive answer to the contention that this amendment prohibits the legislation in question lies in the fact that the amendment is a limitation only upon the power of congress and the national government, and not upon that of the state."
-- Presser v. Illinois, 116 U.S. 252, 264-66 (1886)
"Since we hold that the second amendment does not apply to the states, we need not consider the scope of its guarantee of the right to bear arms."
-- Quilici v. Village of Morton Grove, 695 F. 2d 261 (7th Cir. 1982)
And many more, especially from the Ninth Circuit.