I think you'd be due the business class upgrade amount if you actually paid it - was it collected from you? Then, yeah, the way I read it you have a claim for 75% of the original ticket price - I'm not sure if going segment by segment matters. You're not due anything else because you weren't delayed in the end.
How I'd tackle it depends on who the ticketing carrier was. Did Delta sell you the ticket, or did Alitalia?
I'd file a claim first with the carrier for both pieces. If you don't get comped for the business class upgrade (and actually paid the money) I think you have a pretty straight forward chargeback with the credit card you used to pay, and since that fee would have been collected separately, should be easy enough.
For the 75% of the ticket for the downgrade, be prepared to fight back and forth. Delta might be easier to work with than Alitalia, so I'm kind of hoping they're your ticketing carrier. If it's Delta, start here:
https://www.delta.com/content/dam/de...ed-Kingdom.pdf
I know it says UK, but the EU regs apply, and the contact info will be European but English-speaking. I'd start there rather than calling Atlanta.
Best of luck.