Nothing directly reward reformulations. But the global answer can be rewarded by user feedback. Yes this indirection still seems like an issue.
What I would do instead of this strategy would be to cluster extremely similar/other formulations of the same question by different users and then store for each frequent common question a list of reformulations (user generated). Of course the list would be based on the profile of the user.
I do not answer How similarity/identicality of user formulations would be determined by I have a couple of heuristics in mind.