Personally, I agree that wizarding Britain is so corrupt as to be totally hosed. Unless someone develops a backbone, and Dumbles does seem to have eliminated most, if not all, of the possilbe candidates, they're going down without a leader to challenge them to reclaim their country. I hate to say it, but their best chance might be Tommy-boy over-reching and annoying the non-magical government; they have the potential, possibly with the aid of some muggle-born who went back to that world, to defeat these terrorists of Tom's. Failing that, some other country would have to give enough of a damn to make the necessary heavy intervention and, unless firmly threatened by Tom, I rather suspect the MoM has done wa superb job of annoying its contemporaries in other countries. Of course, non-magical intervention also carries a huge risk of fully exposing the magical world and I could see Britain's magical neighbors possibly collaborating to try and stop that; though I question how successful they'd be. Every successful resistance (guerrlla warfare) group starts with folk holding strong convictions and willing to go the distance to back them (Robin Hood and Zorro fit this as do the members of the Resistance in occupied-France and other countries during WW II). I don't see this mind set in wizarding Britain, save in the folks Dumbles has allowed to be killed off, and I think the British magical society is now SOL.
All it's going to take, is people to gather together and say "It's time to fight back. The wizards just haven't seen enough. The purebloods think they're untouchable, and they control most of the wizarding world. The rest of them just allow the ministry to think for them. There would have to be a direct threat to their standard of living before they bestir themselves. Voldemort has made a goal of eliminating all mudbloods'. What if he murders a family that is well respected in a town. That's usually when people usually decide that they could easily be next. Nothing bestirs the soul to action like the threat to their children. In addition, every witch or wizard carries a deadly weapon at all times. Even if Voldy has his troops confiscate the wands, there are a few who don't need them.
If the muggles get involved, then the wizarding world is doomed. No sooner than they are revealed, (and if that happens the muggles WILL get involved (See Sunset/Sunrise over Britain' by BobMin)they will begin a research program to isolate and neutralize the magical quotient. Once that's done, all it takes is a bullet.
I was actually thinking of the colonial response to the British abuses, during the American revolution, and our defeat at the hands of the NVA/VietCong/ARVN in Viet Nam. They used the same tactics against both the French and the US, telling effect.