That's not much time in Rome. I'd probably take the extra day there. Best to save Stockholm for when you have more time, IMO, but always good to have a happy wife too!

The long train ride from Venice made me cringe, I've only been on overnight trains that long (last one was Vienna-Venice coincidentally). As you probably know, you could hit the main tourist spots in Venice pretty quickly though if you go that route.

One last thought. If this is about fitting another city in, what about flying Rome to Cologne, and driving or train from Cologne to Frankfurt? Cologne is a city you can visit pretty quickly, focusing on the cathedral and surrounding area. It's not Venice or Barcelona though! The train station is right next to the cathedral square and hotels too.