Just out of curiousity, could you give an abridged account of how the HRE has held onto Italy?
Oh blimey, now you're asking...
Imperial control over Italy would have fallen away as normal through the investiture crisis and civil wars.
Olaf the Great then campaigns in Italy, mainly to oust the pope and get an imperial coronation.
France is pretty much an irrelevance at this point but North Italy still descends into chaos over succession crises in Mantua and Naples (which I'll get around to writing at some point) which draws in the Empire, Aragon, Venice and Hungary. Austria eventually pacifies the northern half of the peninsula, if only to keep Aragon out.
After the Fifty Years War Milan is upgraded to an electorate putting in at least some Italian balance to the Empire and Naples accepts Imperial protection to avoid another disastrous succession crisis.
And that's about it. But you shouldn't really think of the HRE as anything more than a vague defensive alliance by this point. There's no economic integration, no real 'Imperial' army, the emperor has little discretionary power. It only really scrapes along because there'd be a free-for-all over the little states if it collapsed.