Imagine that a monster would normally do 1000 damage to you and you 2000 to them with a given attack.
If you cast Sap Mark on that monster it now does only 800 damage and takes 1600.
Same if instead you had cast the mark on your character.
If you have it active on both the damage becomes 640 and 1280.
Each player has their own yin-yang balance which at the start of the battle is neutral (+0).
For each target that a yin spell hits it gives a +1, each target of a yang spell is -1, at most +/-10.
Yang/paradox multiplies it by -1.
Yin spells get weaker the higher this value is; at +0 they have 100%, at -10 135%, at +10 15%.
Opposite for yang. So yes, alternate them.
You could also tank your score by doing an aoe yin spell first and then enjoy multiple turns of strengthened single target yang spells.
No, it gives the same resistance as if the correct status resist relic was equipped, so it only lowers the stop status build-up.
Normally M-Cannon "empties" a field icon per target on use, e.g. 5x fire becomes 4x fire + 1x empty.
Or if all 5 are already empty 1 icon instead becomes a broken icon.
However if the target is heavy instead of empty the icon becomes immediatly broken, so 5x fire would become 4x fire + 1x broken.
Empty icons get just moved along like all other icons and can be refilled eventually.
Broken icons are unmovable and stay until the end of battle.
Both of those oppose all elements, same way a light icon reduces fire damage.
However broken icons additionally hurt the power of your heal spells.
Those broken icons can be good or bad depending on how active you manage the icons; e.g. if you rely on summons this is very bad since those get a much stronger effect from matching icons.
If you tend to lose control of what goes on with the icons you can produce 2 broken icons and then not worry about bosses producing detrimental field combos.