> 1. Почему в русских школах преподают, что обед это dinner, а ужин это supper, когда на самом деле dinner это ужин, а слово supper вообще не используется.

Ну вот не факт, кстати, что всё ещё так преподают, всякие ланчи-бранчи уже достаточно сильно укоренились в массовом сознании.

https://en.wikipedia.org/wiki/Dinner

Dinner usually refers to the most significant and important meal of the day, which can be the noon or the evening meal. However, the term "dinner" can have many different meanings depending on the culture; it may mean a meal of any size eaten at any time of day.[1][2] Historically, it referred to the first meal of the day, eaten around noon, and is still sometimes used for a noontime meal, particularly if it is a large or main meal

https://en.wikipedia.org/wiki/Supper

In England and much of Canada, whereas "dinner", when used for the evening meal, is fairly formal, "supper" is used to describe a less formal, simpler family meal. In some areas of the United Kingdom, "supper" is used to describe an evening meal when dinner has been eaten around noon. In some northern British and Australian homes, as in New Zealand and Ireland, "tea" is used for the evening meal. In parts of the United Kingdom, supper is a term for a snack eaten after the evening meal and before bed, usually consisting of a warm, milky drink and British biscuits or cereal, but can include sandwiches.[citation needed]

Хотя для меня, даже не в англоязычной среде, "supper" скорее ассоциируется с "the last supper", чем с ежедневной едой.