Coding style to minimize combinational path delay?

I didn't bother opening your file but assuming those decodes are mutually exclusive then Quartus should hopefully remove the priority scheme for you. I would get into a habit of just using case statements when you don't need priority though since you are at the mercy of your synthesis engine if you just chain a bunch of ternary operators together.

That said even with the priority removed that's a wide mux so you should consider pipelining it if you need more speed. Luckily a lot of your inputs are hardcoded so those will result in more packing into the lookup table. Probably the best spot to pipeline it would be after all the contant inputs are muxed together so that you can provide a registered copy to the secondary mux that combines it with the non-constant inputs.