In the USA whatever room has the toilet in it is called the bathroom or the restroom. Toilet will work also along with several other words like john, head, facilities, lavatory and others. Bathroom is by far most common followed by restroom
I don't know if Zamenhof that English word in mind at the time, but it's certaily possible. (Necesejo is one of the words in the Universala Vortaro, so it's pretty old.) But as for it's "derivation", it's quite natural Esperanto regardless: neces-ej-o.