Reporting back, just in case anyone else needs to do this or something similar in the future.

06.12 Trains: Walchwil to Zug/Zug to Arth Goldau (this saved us leaving home earlier and 40 mins waiting time at Goldau)/Goldau to Lugano - absolutely beautiful journey. Then bus from Lugano to Malpensa airport. All went without a hitch and we actually got to the airport early! I would never, ever consider undertaking such a journey with so many changes in the UK.

And thanks again for all your helpful suggestions.