Following discussion w/ Jason Grisby at WRI, rather than edit the Q w/ additional detail, the conclusion is that composite `Keys` such as lists of strings, interferes with the above-mentioned named slot access:

    data = <|"a" -> 1, {"b", "c"} -> 2|> // Dataset;

    data[#["a"] &]

    

> 1

while 

    data[#"a" &]

>     Failure[Function, 
>     Association[
>      "MessageTemplate" :> MessageName[Function, "slot1"], 
>       "MessageParameters" -> {"a", Slot["a"]& , 
>     TypeSystem`SkeletonForm[
>     TypeSystem`Assoc[TypeSystem`AnyType, 
>     TypeSystem`Atom[Integer], 2]]}]]