I can't answer this question definitively, so please take this answer with a grain of salt. However, if I were a DWP DM, or a welfare adviser working with a claimant I'd see it this way:
The one month limit applies starting from the date of the change of circumstances which gave rise to the possible SDP award, or the date the claimant notified the department of the change if that date is later. In your case, the change is your award of DLA MRC, so you could not seek backdating for more than one month before the date you informed the DWP of this award.
But the current problems are not of your making. You did inform them and request the correct form, and since a) the DWP won't make the award without the form; b) You informed them in good time and asked for the form; and c) they haven't sent it to you then I'd say you have a case for backdating at least to the date you originally told them about it.
The computer system might not be happy, and this is something I kinda mentioned earlier - the fact that an IS10 has been issued is something that should be specifically recorded in the system, and probably something that the regular contact centre staff can't do. It's supposed to be recorded in order to avoid problems like the one you're now experiencing.
Anyhow, as I said, don't take this as Gospel - I haven't worked there in a while now. But my feeling is you should be OK to backdate for a couple of months at least.