Shooting through/over obstruction

A hill is a pretty solid obstacle, so you need something with a very high arc of fire to fire over it, and that kind of indirect fire is very different from regular fire and not likely to hit unless you fire very many rounds. Dense forests are really not something you can fire through either.

For things that can be fired through, there are a few games like Advanced Squad Leader that has rules like a 1 die-roll-modifier per hex the fire goes through (but if there are more than 6 hexes or something fire is completely blocked instead). But very few games bother with that since it adds complexity and LOS rules are usually complex enough without that, and it is easier to just not have terrain like that but only have terrain that either completely blocks or not fire/sight.

Yep this was covered that in the previous talk about this. Simple die roll modifiers based on how much exposure there is versus complete exposure. This is staple of combat games. Math Anxiety is so high now that even this gets complaints but its still the most logical and simple way to account for such cover.

Realistically, you can absolutely get good cover behind snow, you just need a lot of it to stop bullets, like a few meters or so if I remember correctly. If you are in a cold place and have to entrench you typically do so by shoveling snow instead of trying to dig down into the frozen ground (source: did military service in northern Sweden).

Flame into woods is probably not much unlike other weapons, so similar die-roll modifiers probably. Although if things catch fire it might cause some secondary losses (I am sure ASL has rules for that...).

Gas is probably a lot less affected by terrain or line of sight if you just fire enough of it so you can saturate an area. It might make sense to ignore terrain modifiers for gas.

Yes, like a river. It is gelatinous and splatters when dropped from bombs. When ignited burns more liquid like and flows like lava. Taking cover from it is not likely at all.