1) An undertow isa beach wave usually only dangerous for little children considering it's too small to pull an adult offshore into the deep water, whereas a rip current occurs in the chest-deep water and can be hard to escape even by an adult.
2) 'Burn in' is an idiomatic phrase, hence the 'in'. It means 'To engrave or etch something, such as marks or letters, on a surface by the use of intense heat' which suggests that the story had an impact on him, and not literally burnt something in on him.
3) I don't think there is a differenc between the two.
4) Accomodation or lodging is usually used in the context of travelling, you are assigned accomodation when you go to a different country for a trip or even leave home for prison, whereas housing is generally a place to live, in which you intend to stay for a longer period of time.