Qantas operate the DPS-DRW flight, twice a week currently. You are correct in getting a confirmed reservation on that as if it is withdrawn QF will get you to DRW from DPS without you having to get reticketed at your own cost etc.

I think you can have two stopovers in continent of origin. I am not sure of your home in Italy but instead of going MAD-LHR why not go MAD-FCO-LHR or wherever is close for you that IB have flights to. You could then abandon the FCO-LHR sector and go home.