I don't know if I can shed any light on the reasons for the snippets before the dancing, but many of the names of Cotswold dances which aren't traditionally done with words suggest the tune comes from a song air which was popular at the time it was first used for the morris. Songs like "Such a getting upstairs I never did see" obviously gave rise to the dance tunes for "Getting upstairs" - as this was a silly music hall song from the 1800s it is obviously not as old as morris dancing itself. "The Gallant Hussar" is another obvious example.

I get the feeling that most morris tunes were just popular songs or airs at some time or other, were absorbed at some point or other and often bent to fit the dances because people knew them anyway.

And Trunkles means Trousers. I don't know of a song of that name though.

Cheers

Squeezy