Thought I would quickly follow-up on this now that I took the flights.

In the end I decided to pack much more conservatively and took a cabin-size suitcase only, rather than risk finding out what the charges would be at the very last minute. Of course, at the check-in desks at both ZRH and FNC the staff turned out to be extremely keen for me to check it in (for free, to the final destination) since the flights were rammed. ;-) Not sure what would have happened had I turned up with a larger case.

Checking in to these flights was interesting. For the ZRH->AMS->FNC journey, I could check-in online and choose seats for the Transavia flight from any part of the plane for free (including front-row and emergency exit seats which are usually charged for on an HV code). I was only given a "check-in confirmation" however by klm.com, and had to pick up the boarding pass from ZRH. For the FNC->AMS->ZRH journey, it was not possible to check-in online (nor reserve a seat in advance). At FNC, I arrived early enough for the Transavia agents to give me an aisle seat near the front. They were not allowed to print the boarding pass for AMS->ZRH however, but this was easy enough to pick up from a self-service transfer machine at Schiphol.

So, overall, the journey was fine, and it was nice to be able to select a "premium" (budget airline!) seat for free on the outbound journey. But frustrating that I couldn't check-in online or reserve a seat on the way back. Also quite frustrating that nobody at KLM could reveal the cost of (proper) check-in luggage in advance of the flight. Of course, this problem is solved with Skyteam Elite status... so maybe the solution is to get that first. ;-)

Thanks for the replies by the way. And Funchal was great! :-)