Friends, hear was our resolution - and many, many thanks for your thoughts and comments.
I spoke yesterday to Viking General Customer Service, who confirmed we already did have Viking Air Plus with our reservation, which I did not know for sure. Therefore, we were still able to use that service even though we are under 30 days from our reservation. (I may be wrong, but I thought from the website description that if you didn’t already have ordered it 65 days out, you no longer had that service option.)
Today I spoke with Viking Air Plus. The rep was extremely polite, if a bit skeptical of my late request. I did pull up on my own our preferred flight modifications and then requested to see if they were available through Viking, versus having him suggest flights, although I think he would have gone through the options verbally if I had preferred.
In the end, I was able to add a connecting flight from our city to our original ORD to CDG reservation, then change our entire flight home to connect through DC (IAD) to home. The fare change differential fees were $172 each person. The Viking change fees were $350 each person. Worth it to us, given that this is otherwise a very generously gifted trip, but I understand these fees could easily have been avoided. According to my MIL, her TA told her we had to fly from Chicago, but two Viking reps told me the trip through our home city would have been the same “included air” deal my in laws paid for within our trip costs had it been designated as the destination city for us at the outset.
If anyone wants to add thoughts onto this, please do, but I want to thank everyone again!