This thread has some examples of calling a method with complex inputs. I don't have a server with methods with complex input arguments set up to test this with and write a code snippet against right now.
Yes. Though we might be able to add some new system.opcua scripting functions in the future for OPC UA specific things like this. I've avoided exposing any kind of functionality involving browse paths because the way syntax requires you to specify namespace indices and reference types is non-intuitive to anybody who isn't pretty familiar with the OPC UA spec and it would likely just turn into a support hot spot.
It might be interesting to capture this browse traffic from Ignition in Wireshark as well as get screenshots of the references panel in UaExpert to see what kind of references each node has at each step. HasNotifier and HasEventSource references are removed from browse results before they are returned. Not sure if that is relevant here, though.
edit: Ah, Methods are not returned by the OPC browsing either. Probably because Ignition can't do anything with them in the QC or OPC Browser. But maybe they should get left in so scripting can see them.
I think overall you'll find right now there are a lot of rough edges when trying to use any of the more advanced OPC UA features in Ignition. OPC UA has a more sophisticated and capable data model and type system than Ignition does.