All the things you mentioned I don't care about, because 2 and 3 are only some gui stuff and I don't play with aggressive retaliation, but I'm just confused in general when it comes to available options. Lots of them I can simply turn off if I'm here only for realistic accuracy and nothing else, but then there are some I have no idea how to configure to preserve the original x-files enemy behaviour.
For example (I'll talk about options I found here:
https://mod.io/g/openxcom/m/brutal-ai), "targeting behaviour for Brutal AI" - x-files has the sniper-spotter mechanic, so I guess choosing option 1 (vanilla behaviour) is out of question because it allows enemies to only fire at what they personally can see, same for option 2 (because sniper-spotter allows enemies to fire even without seeing anything I believe), so I guess the closest one is 3, but I'm not sure if it's exactly as in x-files. Note I'm not seeking to increase the difficulty further, I just want to be able to fire at what I can see and to see bullets travelling at realistic trajectories. Another things are "aggressivenes-" and "intelligence-mode" as well as "sneaky ai". No clue how they relate to x-files behaviour.
Another thing I noticed is that while "brutal ai" option can't be toggled, the description mentions being able to set it to false via config file. So maybe that's what I should do to get the results I want, turn brutal ai off via units.rul editing and leave realistic accuracy on? But I prefer to ask simply because AI-related changes aren't something I'll be able to instantly spot to tell whether it worked out or not.
Edit: I noticed there are also things like "ai bug hunt", allow explosives on turn one and so on, I really feel lost with what should I set to what and would be thankful for some insights.