shavua tov,
1) tnai kaful or "double tnai" is one of the characteristics of the tnai made by BG & BR. They elucidated not only what the maaseh's status would be if the tnai was fulfilled but "doubled" up with language that delineated what the consequence of not fulfilling the tnai would be.
2) I understood that the maaseh of settling permanently in those lands was contingent on fulfilling the tnai of participating in the conquest -- that might be what you were saying as well....
3) I think the Rosh is implying that the BG & BR innovated the notion of words having enough gravity to uproot the significance of action. The Rosh seems to feel that this is a chiddush and not intuitive prior to an encounter with Torah values -- at least that was my take from the excerpt.
Also,
thanks Tal for the link...looks promising!