That makes a lot of sense! Indeed, it explains why Mitchell has not overtly done anything sooner: his main role is to see what SIS is planning, and influence M to manoeuvre Quantum's other agents into place as necessary.

M probably realises this very quickly, which is why she is so upset with Bond for not bringing Mitchell in alive. As is clear in the rest of the film, she now has difficulty trusting anyone in SIS.

(If so, this is a classic example of what TV Tropes terms Fridge Brilliance.)