Have clinical trials ever shown that LDL reduction prevents CAD or death?

Yes, essentiall ALL. Starting with trials in the early 1970s & becoming far more dramatic since 1995 (4-s trial using zocor (simvastatin) was reported 1995), essentially all clinical trials showing reduction of either LDL particles (most reliable, see: NMR particle test) or estimated ldl-cholesterol, have shown reductions in all cause mortality, mostly due to cv events plus a trend to lower cancer rates. >30 human clinical trials.
Yes. Clinical trials with drugs that lower LDL have shown a reduction, not elimination, of cad.