To the OP: at least in the "open" materials, you seem to be completely in the clear. I don't recall anything off the top of my head in the MM3 or MM4, either (but that's not a guarentee). Overall, though, I think you're right: that sounds like a great deal. What's the price?
And yes: your weapon enhancement would certainly cut through creatures immune to magic of any kind - they are not "immune to force," but to magic spells.
The only thing that seems like it would confound you is a caster using a spell specifically that made it "immune to force," but that's a pretty specialized encounter.
Edit: Similarly to the force dragon, Races of the Dragon has a half-dragon template for "non-MM" dragons that includes force dragons. So along those same lines, any half-force-dragon would be immune to force.
There's also a dragon listed in the Dragonomicon that gets a +4 resistance to force effects (Tarterian dragon), but as above, a resistance bonus is clearly not "immune." Still looks like you're doing good...