As I understand seasons/weather, these change dynamically in the campaign map. Also, when you enter a scene (such as a battle) its season/weather is set based on that in the relevant location on the campaign map. However, once in the scene the set season/weather does not change dynamically within the scene.
27. Will we be able to simulate the flow of time in a scene without forcing a re-entry? (Morning-Noon-Evening-Night)
The atmosphere system is not designed to update in real-time, so you cannot simulate that without stutters. Maybe you can darken the screen, change the atmosphere and then do a fade in
https://steamcommunity.com/games/261550/announcements/detail/1464090667824385857
On the coalition discussion, Armagan said the barter system could be used to offer any deal to anyone. However, to avoid ridiculous outcomes the price for deals that run counter to an individual’s interest will be prohibitively expensive. Accordingly, I can’t see us bribing Lords of another faction to conduct acts of war against parties at peace with that faction.
Under the hood, the game takes what might be called a rather materialistic approach: In the world of Bannerlord's barters, everything has a price; the game internally assigns a numerical value in the game world's currency --denars to every possible item in a barter, then uses this monetary value to evaluate whether a deal is good or not. Something to be careful of, with this kind of system, is the potential of counter-intuitive results arising: For example bribing your arch-enemy to join your faction, if you pay the right amount. In practice though, this is a non-issue. The system will typically assign such exorbitant values to decisions of this sort, that these will be extremely difficult, if not altogether impossible, for the player to attain.
https://www.taleworlds.com/en/Games/Bannerlord/Blog/12
I expect the nearest we will get to an active coalition (as opposed to coincidental but uncoordinated ones were two parties are independently at war with the same other faction) will be the ability of a faction to hire one or more minor mercenary factions to add to its war effort.