Nick, I took the liberty of copying your initial post & sent it to a friend of mine. Here is his reply:

Hi Nigel Wow lots to answer. You are on the right track re keys and what whistle. Often the cross key is better for songs as it gives more options like extra bottom notes below tonic and still often enough up top if you get my drift. All depends really on where the melody sits. I will use an alto G often for a song in C. You need to be careful with soprano whistles as they can get a bit scary up high with the female voice. My Caroline doesn't like singing with soprano whistles. Mind you my Shearwater Brass soprano whistles she is more happy with for some reason. I actually make E, C# and Alto Bs if you are interested. You name it I can do it. It can be frustrating when you have missing notes on a whistle as it's often the one you need. Hope this helps. Happy to talk more but as I said I can make strange key whistles. Cheers John.

The John in question is John Bushby, whistle maker, luthier etc, etc. His FaceBook page is fascinating. He is also a fine, folk musician in his own right.
                                                 Musically,
                                                                Nigel.