Thank you SusieActually if you read the FAM extract, the onus is more on them to prove it was willful - ineligibility is actually a big deal for US visa officers to decide and it’s not something they randomly determine without being sure its applicable. I cant guarantee anything obviously but I honestly think you’ll be fine. If it had been something like deliberately concealing a serious criminal record it would be a different matter.
Leading on from that, for the question on the ds260 form, of have you ever been denied a us visa, been denied entry, or withdrawn at a port of entry, i interpreted them all to mean at the US port of entry, but do they mean only the last part is at port of entry, and the first two are anywhere, and thus me being denied boarding by airline staff would count as a yes to this question?