I think that you said it well: nothing is more sacred than the protection of informers' identity, especially for an intelligence service whose mandate is to recruit such individuals in order to be better informed. As soon as you undermine that protection, you're out of business.
I don't think it is necessary for the committee of parliamentarians to know those people's exact identity. That won't prevent it from doing its job, as long as it knows what the key elements of the case are. I believe that this principle applies both to the world of police and to that of intelligence. I completely agree on that issue.