Azizi, I think your novel has some very fanciful ideas that are not really based on fact - at least, not in Padstow.
The verses you quote don't ring any bells - the words of the May songs have changed a little over the years but not that much.
The Blue Oss song is here.
You are right about the instruments - maybe 150 years back there was a pipe and tabor playing the music but apart from 1972 and 1990, when I played recorder in the Blue Oss band, mostly the bands have a lot of squeeze boxes of varying sorts and sizes, and lots of loud drums going Boom ba ba boom all day long. There is a huge crowd and anyone trying to play a flute is likely to get their front teeth knocked out. By the end of the day, my recorder had been bumped into so many times, my lips were swollen and blood stained.
Some of the description in your book could relate to the Halanto at Helston on 8 May - the youths with sticks - branches, waving in the air and the girls with garlands in their hair, but it doesn't really sound like Padstow.
"the rabble of grotesque masks and caped bodies was drawing tighter and tighter, threatening them with pointy sticks, and the shrieking voices were all but piercing her ears..."
There's a little about Helston at this site.
There are photos here.
But the story sounds a little sinister - almost as if there's about to be a human sacrifice - 'virgins given to a sea monster' and in both Helston and Padstow, the atmosphere is very open and happy and not at all sinister. Yes, in Padstow there is death and resurrection but it's the Oss that dies, not any of the young girls and in the Halanto there is mock fighting - as St George fights the dragon and when St Michael chases off the fiend, but it's all good wholesome fun.
I think the author has combined a little bit of reality with a lot of imagination in order to make an interesting story.
Robyn