Given the original description, it was clear it was going to be testing of an alpha or beta version, with a fair bit of development still being planned, and I have no problem with trying it and trying to provide some feedback. However, I'm not a great tester.
Overloading: Yes, the high level language concept of discriminants (as it's called in Ada) to determine how overloaded data should be interpreted isn't supported by High Level Assembly (a.k.a. C(++)) unfortunately. XML does have the capacity to describe it though, but most users of "XML" seem to ignore the built in features and use home brewed attributes instead, nor are XML schemas used to any extent to describe the legal syntax (that could then be validated to catch mistakes).
I believe it's necessary for the tool to load only the data that's to be displayed or starting would take forever and use up a huge chunk of memory, but there's a difference between laziness and complete amnesia... Remembering where you were is a kind of implicit expansion (with checks to prune references that do no longer exist). I agree this is a lot easier with the logic of gui/gm-editor, since you'd only need to keep track of a single expansion path, but it should definitely be doable with a reasonable effort.
The logic needed is probably similar to the one needed to open a given path, but with some additional complications, so I'd probably tackle the simpler problem first.
The ability to use an old and a new copy of some data and compare them, ideally with highlighted differences, would probably be very useful for field identification, as you'd be able to see what your in-game change resulted in (at that location) at a glance. Windows could possibly start outdated on a reload, with options to refresh or compare.
I agree the ability to follow references would be quite useful, but it can be noted that a lot of references are indicated only through field names (such as "hfid"). However, if there's a tool that can make intelligent use of such references, the drive to update the XML description with those attributes ought to be enhanced...