One thing is that the figures were sort of the "character sheets." Their representations, as well as our shared knowledge of the Star Wars movies and the characters in them, provided the definitions of the characters terms of personality and capabilities. Another thing is that we each played more than one character. We controlled a wide range of figures in our play, and these characters were mixed heroes, villains, NPCs, and mooks distributed between the two of us. Finally, there was no GM. We played adversarial roles against our own characters and each others'. Things happened merely because we wanted them to happen. Each player had equal authorial power.
I went pretty much right from playing with toy cars and action figures into role-playing and, in fact, my earliest homebrew rules were for Star Wars action figures and my earliest role-playing games had no GM,the players played multiple characters, and we played adversarial roles for each other. As such, I have a pretty good idea of what I got from using a rule system and playing using the conventional GM/player structure. Can all you describe be done in an RPG? Yes. I've done it. It's how my earliest Traveller games were played. But, personally, I've been there, done that, clearly remember what I used to do and like what I do now better.
In fact, one of the reasons why I've never seen role-playing as competative or about getting stuff is that despite playing tons of board games as well as a kid, I quickly associated RPGs more closely with the cooperative imaginative play with action figures, toy cars, and Legos and not competitive board games.
The only goal was entertainment. Immersion was not a goal, and may not have been attainable in the circumstances.
I would think it would be difficult. What I did see, with both action figures and those early RPGs, was players who would essentially create themselves (or a version of themselves) to stick into the game for avatar play. This was less common with Star Wars figures but more common with things like Legos, Action Jacksons, MAC figures, toy cars, and other generic character toys that weren't linked to specific TV or movie properties.
It struck me that this is what the Forge is after, exactly this style of play. However, instead of starting with two people who want the same goal and merely want to play a story, they seem to start with the concept of two people who can't agree, who have some desire for oneupsmanship and need an outside arbitrating force of rules and a GM to then force them through those rules and arbitration to tell an appropriate story.
I sometimes wonder if I'm the only one who remembers doing this as a kid because they make it sound like they've never seen or done anything like that before.
Another thing to think of: Is playing with action figures an RPG or not? Why so? My attitude is that it is not. Playing with the figures is about play. It isn't about competition, which is why RPGs need a GM to arbitrate matters (sometimes to provide outside competition to the players) and rules to facilitate things and provide a framework and boundary for the competition.
I've never looked at the GM as being in a competitive role or the objective being competition. Like I said, I've always seen role-playing games to be a close cousin to the sort of action figure play you described. As such, I think they can be a form of role-playing game, though I think they differ from what we normally mean by that term in some very important ways.