And im confused once again.

So ... if you want to have advantage, you cannot have any disadvantage, and it dont matter how many sources of each you could aply, they all count as 1 ... corect?
So how can you aply Snake Attack, if its description specificly tells that you need to have Advantage ... and litteraly "cannot have Disadvantage" ?
If you have at least one factor that would apply disadvantage to your attack roll, and at least one factor that would apply advantage to your attack roll, then you
Do Not Have Disadvantage On Your Attack Roll. Your attack roll is a normal roll, made with
neither advantage
nor disadvantage.
You can apply your sneak attack under two different conditions. These conditions are separate and you only need to satisfy
One of them, not both. The conditions are:
1) You
can apply sneak attack
if you have advantage on your attack roll.
2) You do
not need to have advantage on your attack roll, and
can still apply your sneak attack,
if there is a creature hostile to your target within five feet of it (usually your ally), as long as you
do not have disadvantage on your attack roll.
This means that:
If you have a factor that would impose disadvantage on your attack roll (such as being at long range), AND a factor that would grant advantage on your attack roll (Such as having your target lit up by faerie fire), AND your target has one of your allies threatening them... then You CAN apply your sneak attack, because you
Do Not Have Disadvantage On The Attack Roll, and thus satisfy the second option for applying sneak attack.
(Once again, I suspect there might be a language issue between us here, Rag - let me know if this is still not making sense to you, no offence is implied)
==
As for the teleporting goblins... Larian wanted us to use their high ground mechanic a lot. Then a lot of players did. Then it turned out that doing so made the AI fail to pose any kind of threat, because it couldn't deal with it. So then Larian added in special "enemy Only" tunnels that the AI could use to negate, at will, the mechanic that they encouraged you to abuse in the first place. It's apparently "Working As Intended".