I have no experience from these kind of cameras in real life. But in the article you reference it says:
"it is a good thing that the thermal imager can see through fog to a certain extent."
Sounds to me it is not totally unaffected. Also, the thickest fog in arma is quite thick and would deliver a massive amount of moist in the air. Seems reasonable to me it has a severe impact on the way heat can be read at a distance.

EDIT:
Found this folder from FLIR which backs this up as correct behaviour:
http://www.flir.com/uploadedFiles/FOG_techNote_LR.pdf

"Humid air acts as a “shield” for infrared
radiation"

I believe this is by design. The movement of the crosshair is to indicate that a certain object is blocking the line of fire.
I haven't noticed or thought about any resizing of the actual crosshair in the process though.

I can't remember where I read it though.

One thought would be to have a video option that lets you set the resolution of your center screen as to indicate the boundaries for game menus and editor ditos.

E.g I have three monitors with resolution 1920x1200 setup in eyefinity. With bezel correction I'm running at 6048x1200. Normally Arma3 sees this as every screen beeing 2016x1200 which is obviously not the case.

If I could tell the game that my primary monitor estate was the center 1920x1200, and that the game needs to keep menus within this boundary I think that would help. It is of course nice that you can drag HUD elements around and place them wherever you'd like, this proposition is only for the fixed menus.

I've seen the same. players who don't have the game in cache might or might not see the tasks and diary records. Mostly not.

Second start of the missions when it is in cache they all see it. I'm using the "Grouped option" for the tasks.

I got the exact same issues.
There is a way to adjust it slightly with .ini file editing, but the result never turns out perfect.

This one is identical to #issue #0002542