I think the dynamic you describe where betting for real money “incentives [people] to try to set the most unfavourable odds they can for the other party” is correct, but also good. Two rational actors will only engage in a bet if they believe it is +EV for each of them. In a zero sun bet, this is only possible if each actor assigns sufficiently different probabilities to the event in question. So actually in the case you described about the £10k bet, useful information was conveyed. You demonstrated that you were not sufficiently confident in your claims to offer better odds (ie. you didn’t think offering better odds would be +EV for you), which is fine. Obviously if you aren’t a frequent bettor, you have to be more worried about variance (getting unlucky and losing a bunch of money on a good bet that you’ll never recoup), but in general bets are actually good at teasing out someone’s probability distribution for an event occurring, and the dynamic you describe is an important part of that.
Regarding more examples, I think that any action which someone could say "I would do this but what difference is one person's action going to make" is a candidate for a good campaign. More examples I can think of mainly relate to conservation (energy, water etc.). I also think this platform could help power boycotts of companies which could be a very powerful use (but also with a risk of becoming dangerous as pointed out by Ramiro). I actually think that this alone would be a very powerful use for such a platform.
And regarding my second issue, I think for the platform to have a chance, it would have to go viral and somehow maintain a user base after that. I agree that acquiring a user base dedicated to making actual changes in their lives would likely be the hardest part of the process.
Also, thanks for linking to that report. It seems to advocate that a grass roots platform like this could be one of the more effective ways to affect change. I definitely would like to do some more reading on the research in this area though.
To be honest, I haven't considered that at all. I can definitely imagine that if a platform like this were to become popular it could start organizing campaigns that don't necessarily conform with my values. However, I think that with minimal moderation those could be dealt with, but I would have to walk the line between removing truly bad ideas and ideas which I just may not completely agree with.
But this is still a problem that I only think we could even face if such a platform reached a large critical mass of users, which is pretty far down the road.