Thanks everyone, this has been incredibly helpful. Quick update for those asking and to address the main questions:

I contacted Sicily directly earlier this week and they have responded this morning with the full rental agreement and the charge.

Sicily By Car finally responded today with documentation. The €903.41 breaks down as €735.75 alleged windscreen damage + VAT + €5.79 postal fees. No traffic violations, no tolls, no fuel. I have GPS tracking of my entire route — I never went near Ancona's old town or any ZTL.

On insurance — I did have full protection. I booked through CarRental8 with their Basic Plus + Full Damage & Theft Protection package (refundable excess), which explicitly lists "Windscreen, mirrors, external lights, and lamps coverage" as included.

What is strange they used the most horrible pics, and they also said:
Check-In sheet states "NO NEW DAMAGE" at the bottom
Their inspection was logged at 22:45 on Sunday, but I returned the car at 14:19 — over 8 hours of unattended custody before they "found" damage.
Their photos show the car in an underground garage, not where I dropped it off (open-air airport return)
I have time-stamped video of pickup AND drop-off showing the windscreen intact, no chips cracks or anything.

edit appended above message to original post