Is feeding ten poor people with the intention of expiating a broken covenant sufficient for the expiation of a broken oath, or is it obligatory to expiate again with the intention of violating the oath, if it becomes clear that what was covenanted upon was a form of oath?

What you termed "breaching the covenant" is unclear as to whether it necessitates an expiation (kaffarah) for an oath. If it does, then the expiation you performed is sufficient, as specific intention is not required for expiations; rather, the intention for the root of the expiation suffices. So, if you had multiple expiations due, any expiation you perform would suffice for them without the need to specify which one.