When the team has Ada submerge in the drum of honey using only an oxygen mask, they show her using a typical emergency mask. This mask is not seal-proofed and has multiple holes in it allowing for the exchange of carbon dioxide. This would also allow the honey to leak into the mask, thus drowning her.
There would have been an air bubble suspended or moving slowly in the honey from her exhaling had she actually been inside a barrel full of honey.
A simple wooden structure could have been erected over the bubble in less time than they spent using the Jenga style debris removal method.
Walter claims gelled alcohol ( Sterno) burns at 185 degrees.., it is like any other flame it burns hotter than 500 degrees. In fact when using gelled alcohol in a chafing dish you have a pan full of water that the food pan sets in so the food does not burn.