Encumbrance question

[Illuminated One]

I'm a bit at loss with the encumbrance calculations in the game.

1.) If you have a unit that has an encumbering armor and you give it a magic armor with less encumbrance is it normal that the unit keeps its old encumbrance value?

2.) Does being underwater affect encumbrance?

3.) How do multiple weapons affect fatigue? Do you get fatigue for each attack or each round? Are there weapons (like warhorse hoof) that don't generate fatigue?