Further discussion has taken place with my landlady and it seems now that I will definitely be riding both ways - landlady will look after my dog between 20 and 30 April (I may not need that entire time, of course, but wanted to be sure all options were covered). Distance appears to be about 430km taking the Rhein Radweg from here although it could be shortened a bit if I rode directly to Köln from here cross-country rather than along the Rhein.
One chum from here will not actually ride to Spezi but stop at Mannheim (30km before) where his parents live and spend the day with them.
I'm thinking of one day at SPEZI (Saturday?) and the next day probably a short ride and a relaxed look around Mannheim which is apparently really nice before heading back to Kempen. Probably 100km per day I suppose.
This would be overnighting in hotels (although others are welcome to do what they choose of course, if camping appeals).
Peter and Jan, it would be great if you can come! Or maybe if that's too much time out of the working schedule then perhaps ride one way, train the other, if that helps.