Just to illustrate how contemptuous Delta are of their customers, read this:
http://www.airlinecomplaints.org/showthread.php?t=7693
This person's experience, whilst typical of Delta, resulted in arrival at the destination some 12 hours late. They were given a $100 voucher, exactly the same level of compensation you were offered. To offer the same compensation in this situation is insulting and derisory.
You certainly can sue a US carrier in a Canadian court. They provided a "service" to you under Canadian jurisdiction and you can take them to court to seek a remedy. I would not allow them to continue to blame "ATC problems" for the sequence of events which affected you. You are sueing not because your flight was late, but because the incompetence of their employees resulted in a significant disruption to your honeymoon.
You are entitled to reimbursement for any additional costs you incurred due to the delayed luggage. This includes the cost of purchasing additional clothing and toiletries. If you have receipts, you can claim any reasonable amount (Delta cannot legally impose an arbitrary limit).
I believe you also have a reasonable claim for the failure of the Rome to Valencia portion of the flight. Your entitlement under 261/2004 can be found here. As this was entirely within the EU, you may be able to write to Delta and/or Alitalia and demand your rights for this portion of the journey and the associated delays. There was no reasonable excuse for this delay and it amounts to a "denied boarding" or extended delay.
Your rights under 261/2004 are listed here. You might want to quote this regulation when you have an exchange with the supervisor, in relation to the specific problems you encountered in Rome.
http://en.wikipedia.org/wiki/Regulat...g_or_refunding
If you get nowhere with Delta, who also operate a "refuse every claim" policy in the hope you will go away, with a standard canned response, I would encourage you to seek redress via the court. At least there you are guaranteed a sympathetic hearing. Delta's position that an ATC delay at the beginning of your journey indemnifies them from any liability for any subsequent service failure is legally nonsense and as usual, OUTRAGEOUS.
http://www.attorneygeneral.jus.gov.o...urt_Addresses/