Hey Pecan,
I'm not familiar with mid-trip deviations. However, the train from Cologne Hbf (central station) to Paris Nord is great. Just look into Voyage en train vers Paris, Bruxelles, Amsterdam - Thalys.com for schedules and cost. The only problem I have had with Thalys is the inability to use the company card for an online purchase because they require a second party authentication (or something along those lines). For on-line ticket purchase, you can use your personal card and then expense it as an Out Of Pocket item. Otherwise, you can just purchase your ticket at the station.