Monsieur Malacher kindly shared his knowledge with me and I feel obliged to show the answers in the thread:
1st - the combat leader casualties dice sides value is obsolet
2nd - the leader death combat chance is exclusively determined by the model (field: CbtDeathChance)
3rd - each round of a battle a 10.000-sided dice is rolled against the value given in that field
4th - even if the field is "nulled", there is a 0.2% chance each round, that a leader involved in a battle dies - so the default value is 20
These answers only cover battle-related leader deaths. There are still other possibilies, e.g. leader death by sickness, leader death by siege, etc.
Once again, merci beaucoup!