Have you ever travelled with the Ystad - Swinoujscie ferry? This certainly would be a nice way to start a trip...but unfortunately it doesnt make too much sense for your journey I guess. If you take the day ferry you could catch the night train to Krakow, but would already need two travel days, haha...
So...there's probably not an alternativ to going to Hamburg first. From there, you could get the nightjet to Vienna or Zurich: check night train
Have you ever travelled the Bernina railway? blog.rail.cc/zurich-milano-albula-bernina/
You could easily get there from Zurich: check scenic route
So...if you'd go Falkenberg - Hamburg - nightjet to Zurich - Bernina and stay overnight in Switzerland (have a look at the Youth Hostel in Pontresina or rail.cc/en/ospizio-bernina/albergo-ospizio-bernina/l1139
you'd need two travel days.
To get to Diano Marina you would then simply go Bernina Railway - Tirano - Milano - Diano...and you'd miss out on a high speed train trip.
Oh by the way do you know about the new station and alignment in Diano? Recently the new railway line (mostly tunnels) has been opened...
BTW there are no limited seats on TGV trains, at least on domestic TGV, I think there might be limitations for some international lines: blog.rail.cc/interrail-reservation-guide-tgv/
Taking the nightjet to Vienna doesnt make too much sense I guess...since this would leave you with only one travel day to get to Diano which will be quite the trip to do in one day.
If you want to include a TGV and go via France you could look at the nightjet to Zurich, then travel via Geneve to Lyon and stay there overnight. Using your third travel day you could get a TGV Lyon - (Marseille) - Nice and then continue to Diano. Actually this would be quite an interesting option too, allowing you to see some scenic parts of Switzerland (although not as spectacular as the Bernina Railway) and include a night train and TGV.
...btw do you know a Marina from Falkenberg who also did a family Interrail trip some time ago? I remember her writing here on the forum and I guess she also had a Swedish blog about her trip...