I do recall she got a shot at Vader in Splinter of the Mind’s Eye but I don’t remember if it was handled well.
Splinter is generally a strange artifact of the pre-ESB expanded universe. It’s very entrenched in a ‘70s kind of sci-fi writing and none of the core stuff ESB established is anywhere near Foster’s mind. It also has Vader’s defeat coming from him tripping over his own severed arm, so that’s kind of a weird moment.
Yes, that makes sense. That’s why I thought it might be pertinent to the OP’s point in that, in canon terms, it’s a continuation of '77 Leia rather than post-'80 Leia. Like I said I don’t really remember the details (I was kind of ambivalent about the book back in the day) and, if memory serves at all, I feel that Foster pushed Luke’s confidence at the expense of Leia’s prior assertiveness. I certainly don’t remember Vader’s severed arm incident!