What is the ruling on swearing by "wa-nah al-'Azeem" or "wa-nah" instead of "wallah al-'Azeem" to avoid prolonging a dispute, while still retaining some anger, and is this considered a false oath (yameen ghamoos) or does it necessitate expiation (kaffarah)?

Euphemisms (ma'arid) provide room to avoid lying, so you can use them to get rid of embarrassment. For example, you can swear that you are not angry with her, intending by that the previous anger. This oath is not a ghamos oath (a false oath taken knowingly to usurp the rights of others) because you did not intend to swear by Allah nor did you utter it. However, it should be avoided out of reverence for the names of Allah the Almighty.