I am not sure if you were posting a link but it does not appear to work so it is difficult to see the scenario that you are describing.
Also, what do you mean by 15ft/stealth and perception checks are -1 per 10ft so I do not know where you got -5 vs Perception for every 30ft.
I will try to answer some of your questions.
#1 - If the goblin is in dim light or darker and the human cannot see in the dark such as through Darkvision, then the goblin makes a stealth check in the concealment of the dim light versus the human's perception check. If the human fails to see the goblin, the goblin can step up and check out the pot. Once he does so, he has broken concealment and can be seen. Since he did not take an offensive action, it would roll to the next round and normal initiative would begin. Basically, the human was surprised during the round the goblin was sneaking up to the pot and that was the surprise round.
#2 - I cannot see the diagram, but using the same scenario above the goblin makes his stealth check while in the concealment of the dim light and the human makes his opposed perception check. It the human fails to see the goblin, then the goblin can make one surprise attack on the human. Then initiative is rolled and combat proceeds as normal.
#3 - Yes, if the human is now in the dim light, then the goblin can use the concealment of the dim light to make a stealth check.
#4 - Yes, the stealth check from the goblin would be broken by his first attack, but he could make more stealth checks on subsequent rounds and continue to surprise his opponent. It is not wise for non-darkness seeing creatures to fight creatures that can see in the dark while they are in the dark.
#5 - Blur grants concealment and cover or concealment allow for the use of a stealth check regardless of the light conditions. **EDIT This might not be true because of some verbiage in Ultimate Intrigue regarding Blur and stealth. I do not have it handy right now, but I will look into it and see what it says**
I hope that helps!
Question #3+#4: Is it RAW only the first attack which gets the stealth/sneak-attack bonus? Well, I would deny iterative attacks but two simultaneously attacks (Two weapon Fighting), im not sure about…And what about action economy? It’s not intended to use stealth between your 3 attacks in that round, but someone might read it that way (“Action: Usually none. Normally, you make a Stealth check as part of movement, so it doesn’t take a separate action. However, using Stealth immediately after a ranged attack (see Sniping, above) is a move action.”)?
Your Stealth immediately ends after you make an attack roll, whether or not the attack is successful (except when sniping as noted below).
RAW and RAI, only the first attack gets the stealth/sneak-attack bonus. Two-Weapon Fighting attacks are not made simultaneously, and even if they were only the first attack roll can benefit from using the stealth skill.
If you have the skill unlock for stealth then you can benefit from stealth on all attacks you make during your turn at level 15.*
Stealth checks are made as part as of movement. Attacking someone with your dagger is not movement. Drawing a weapon with a move action is not movement.
A 5-foot step and a move action to move are movement, and would allow you to hide.
So if you have concealment/cover, and are already hiding you could make one attack and benefit from stealth, use a five-foot step and hide, then attack once more with benefit from stealth. All during the same round.
The benefit from Sniping is that you're considered to be hiding even when you're attacking, which prevents enemies from noticing your location or using readied actions to hit you when you'd normally be visible.
Question #5: I would really like to be sure about this, with stuff like Cloak of displacement (minor) it would be quite easy to get those sneak attacks.
Here's what Ultimate Intrigue has to say about effects such as those gained from a Cloak of Displacement.
Cover and Concealment for Stealth: The reason a character usually needs cover or concealment to use Stealth is tied to the fact that characters can't use Stealth while being observed. A sneaking character needs to avoid all of an opponent's precise senses in order to use Stealth, and for most creatures, that means vision. Effects such as blur and displacement, which leave a clear visual of the character within the perceiving character's vision, aren't sufficient to use Stealth, but a shadowy area or a curtain work nicely, for example. The hide in plain sight class ability allows a creature to use Stealth while being observed and thus avoids this whole situation. A sneaking character can come out of cover or concealment during her turn, as long as she doesn't end her turn where other characters are directly observing her.
Keep in mind that this is calling out "effects such as", and is therefore not limited to only preventing Blur/Displacement from allowing you to hide.
My own opinion is that this disqualifies all single-target concealment effects from allowing you to hide (unless they are implied to allow you to do so), since that would leave a clear visual in most cases. But I believe you'll meet many different views, as it has always been a debated subject.
The core stealth rules are a poor representation of real-world interactions, since they imply that nearsighted people can lose track of a person in a well-lit, empty and white room. Most arguments start from what the rules actually say (and the designers intended), and what people believe is rationally feasible.