Damn Ben, that is pretty awful.
I would suggest going over to FlyerTalk and putting this up on the Delta forum and see what those folks have to say. They tend to know the airline policies inside and out and may have some tips or tricks for getting this resolved. Also, not sure if there is EU compensation for this (tends to be mostly only for delays) but someone may know. If you have Twitter you could always try the social media name and shame game too. One more thing - next time you call an airline, and I mean any airline, if you are getting confusing/conflicting information then HUATA - hang up and try again. Sounds like you got two halves of correct information from two different agents!
Putting your luggage on SAS out of Paris and into Newark would have resulted in at least a day or two of delay as they would have had to fly it from Paris up to a Scandi hub and then over, and they tend to only fly two flights a day into Newark so depending on when it was, you would likely have already missed the flight for that day and potentially the next.
Finally, infants under the age of 2 would not be ticketed as they are seated in the lap, although they would need to have a reservation I believe for the manifest.
Delta's policy regarding the 10%:
https://pro.delta.com/content/agency/us/en/policy-library/reservations-and-ticketing/infant---children.htmlInternational Taxes & Fees—For infants under the age of two and held in the adult's lap, the cost is usually about 10% of the adult fare plus any international taxes and surcharges, which can be significant.
Delta once left me in Greensboro, TN because we didn't have enough fuel to get to Atlanta... from Chicago, causing me to miss my onward to London. I feel your pain.