Following discussion w/ Jason Grigsby at WRI, rather than edit the Q w/ additional detail, the conclusion is that composite Keys such as lists of strings, interfere with 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]]}]]