Hello and welcome - cant help you with the boil in the bag - never tried it.
I have boiled water in folded A4 and the trick there seems to be don't let the flames above the water level...
Ditto.
Did it many years ago at school. The chemistry teacher showed us during a boring lesson. Folded a sheet of paper to make a tray/dish, paperclipped the corners and stood it on a tripod over a bunsen burner.
Subsequently, in the Army a few times, during those long boring buts, I bet people I could make a brew in a sheet of paper. Same folded paper trick, edges folded back on themselves if no paperclips available, using a hexie burner with a few tent pegs placed across to provide support for the paper tray/dish, fill dish once over the burner............got them everytime.
Digressing slightly, I also remember a fire related incident where a helicopter bubble polisher was 'expertly' explaining about the high flashpoint of fuels and the lower burning temperature of matches. He demonstrated stood next to a gazelle, you could have painted the remains of the heli silver and sold it as modern art. My point is,careful when playing with fire but give it a go.