First - those are QnA thread sorts of questions.
1. When I first started a new game, i chose to not have fog of war but when I start playing, fog of war still enable.
Hard Mode enabled? It forces FoW.
2. I can't shield bash, even though I checked and the Silverstag option is enable
as everyone else, it was disabled long time ago
3. I ticked "Horse speed limitation" but that doesn't happen in combat, even if the horse have 1 hp left, it still run like crazy.
Try to change it in diplomacy settings (not mod settings)
4. Some of my cavalry still trying to unmount in crowd even though I unticked the option "Do not force cavalry to mount" which means they must not be unmounted.
Because reading description of toggler would be nice - it's enabled by default (and this is mentioned) and works fine so you disabled it yourself
5. I don't see stamina bar anywhere.
There is no stamina bar, sprint time and speed are based on your stats.
1. I talked to Khivachis about fighting the sectarians, after i defeated about 5 or 6 of them, I came back and she just gave me money, then she only asked about my progress and nothing more. Now, the sectarian dragon priest just keep spawning everywhere around Reichberg, attacking caravans and villagers, it really is a pain to travel there. Any solution?
They will keep spawning until questline is finished and actually if you defeated 5 parties (you must initiate all fights, not help someone), talked to her and took money, then you need to talk again and start last quest.
2. I did the Venetor quest line where I have to talk to the merchant in Krain about water mill of some sort and he just told me to send his regard to Visavo Austad, after that Venetor faction established and nothing else happen. I don't understand what happened so I talked to him a few time more about the watter mill, same answer, messages about trade routes pop up and Venetor have almost 9000 garrison troops. Big "oof" for me.
Last person who did that was unable to complete the quest. It shows in quest log that you need to talk to Austad iirc, he'll spawn in Venetor in few days.