Should the SetParametersResult SetParametersResult be names singular (SetParameterResult) since it only describes the result of a single set parameter? The service then uses a list of these.

It depends on how you interpret it, because SetParametersResult should read as "a Result of SetParameters", but if you change it to SetParameterResult then it would be "a Result of SetParameter", and SetParameter is not a service that exists. However, you could consider SetParameterResult to me "a Result of trying to set a parameter", where SetParameter is not a proper thing, but a reference to an attempted action.

I'd vote to leave it as is, because one Service uses a list of them, but the SetParametersAtomically uses only one, yet it describes setting multiple services, not just one.

My interpretation was "the result of setting a parameter". I found it unnatural that the "result for set parameters" contains only the result of setting a single parameter.

Without the full phrase it's going to be slightly ambiguous anyway. I don't think there's a "correct" solution.

+1