I've never flown the 421 or the 404. I have a fair amount of time in a Queen Air, they run geared Lycoming IGSO 480s. The primary thing that you wanted to be aware of was to make sure your props were always pulling. You do not want to push air with a geared engine IE flatten your pitch and use your props to slow down. I always treated the big geared Lycomings very gently with plenty of warm up time and gentle power changes. I treated them like they were big supercharged geared radial engines and I never had an issue with them.

It seems to me that I hear about an awful lot of those geared Cont 520s coming apart. In fact I don't think I know a single 421 operator who hasn't had engine problems.

So my original question still remains unanswered. The airplane was on a repositioning flight and presumably light (ish) weight. Will a 404 fly on one engine in Denver at lighter weight? The DA was way down with the temperature. Sounds like this was a sharp pilot. I'm thinking that he had multiple issues going on top of the engine failure.