Alitalia is a bit dis-organized. Each Skyteam airline has their own Frequent Flyer plan and there is little co-ordination between FF plans (some would say none). Thus the terms for AZ might be better (or worse) than for DL and there is nothing you can do (except pick which plan to credit the miles into for a specific flight). It sounds like you are well served with the AZ plan. Only AZ can answer your questions, and the terms for DL have no relevance.