see you are learning the rules,
risk assessment on drilling the lid is: use hand drill as it is safer, goggles AND gloves, a vice to hold the lid (i know risk assessment on the heavy vice)
read safety list for flux and it says suitable to use on copper pipe for soldering, so no risk assessment required as it is fit for purpose, its heating the lid that is risky as we dont know the type of plastic the lid is made of, and again safety first, do all soldering outside and take pipework sections back in and fit with compression fittings (no joke this appeared in an amendment to the EAGA contract when a couples house got burned to the ground when getting their new free heating)