1.

"It’s legitimate for the courts to stop people’s rights to compensation for a tort evaporating simply due to the number of wrongful actors involved. "

I don't think we can say that. It is true that if I wrong you I cannot escape responsibility by pointing at one or more other wrongs you suffer at the hands of other people.  But the question in issue is who is the wrongful actor? A civil wrong is not constituted by negligently firing into woods, or driving drunkenly around roundabouts, or telling people lies. It is constituted by injuring other people by negligently firing into woods, hitting them with your car or telling them lies they believe. 

In the example there is only one wrongful actor, we just don't know who he is.

2.

"I also think this can be justified, sometimes, on the basis that each defendant either wrongfully caused the injury or wrongfully prevented the claim in respect of that injury."

I don't think there is anything wrongful, per se, in negligently preventing you from bringing a claim that would otherwise succeed. If D1 destroys information that P could have used to bring a claim against D2, that is just bad luck.

3.

"It can also be justified on the basis that it prevents the causative defendant relying upon the other’s wrongful conduct so as to deprive the claimant of a right of compensation in respect of a loss. Compare the analogous line taken in Baker v Willoughby, where C is still said to have suffered a loss at D’s hands during a period in which had D not behaved wrongfully, C would have suffered the lost earnings etc anyway. (The explanation of this is not, as some suggest, that C has a right to be free of wrongful conduct and so the relevant counterfactual is ‘What would have happened had C not been wrongfully injured by anyone?' This is not an accurate statement of C’s right against any particular D: C has a right against D that D not wrong C). I think we can also come up with cases of multiple omissions (such as those described in Jane Stapleton’s recent paper in the OJLS) where there simply is no causation by any of the defendants to speak of – and yet we still should say each defendant is liable.

Or compare situations where there are multiple symmetrical minimal contributions to a causal process. Say 5,000 polluters pollute a river, each adding a tiny amount of pollutant to the river. No individual contribution is necessary or sufficient for the existence of a public nuisance, let’s say. Nonetheless each is liable. Even if we say each is a NESS cause of the existence of the nuisance, the contributions may be so minimal as to fall foul of a ‘material’ contribution requirement. Nonetheless, it’s justifiable to hold all liable (proportionally?) in order to prevent responsibility from evaporating simply because lots of people have behaved wrongfully."

All of these are examples of where D's actions have contributed to an outcome but not caused it. Both in morality and law, we can treat those who have merely contributed, but not caused, as responsible. Two weeks ago I cast a vote in an election for a candidate who was successfully elected. I didn't cause his election, he would have been elected anyway (the majority was in the thousands). But I was one of those who contributed to his election. Each of us who voted for him were in part responsible. Perfectly justifiable for the law, tracking morality, to hold responsible some people who have merely contributed but not caused.

That is not analogous at all to the hunter scenario where we cannot show on the balance of probabilities that there was any causal relation at all between any particular gunman and the bullet wound. 

"Finally, I don’t think Rob’s view that if you can’t prove the D wrongfully injured you on the balance of probabilities, you have no claim is defensible on other, independent, grounds. If D1 and D2 each wrongfully injure one of C1 and C2, and each risks C1 and C2 the injury caused, but it is evidentially impossible to pair up the claims, must each C really lose here? (No.)"

Say we have lots of hunters firing into woods, and many victims, and we cannot show from which gun each bullet came from. If one victim claims against one hunter should the claim succeed?

If you cannot show that I injured you why should it? Why should my position with respect to you change because of the presence of other people, with whom I have no relationship, negligently firing into the wood? Why should your claim against me succeed on the basis that I may have committed other similar wrongs with respect of other people?  

Put another way, I have no problem clobbering wrongdoers, but am still unhappy with clobbering people we cannot prove are wrongdoers.

Rob