[RPG] Hide in Plain Sight vs True Seeing

It’s practically impossible (-20 penalty) to hide while attacking, running or charging.

So you can remain hidden while making a spring attack, but the DC to spot you is not going to be very high. If you still manage to win the opposed roll, your target will be denied its Dexterity bonus to AC and will have an additional -2 penalty because you are effectively invisible.

From Skip Williams' column:

If a foe you cannot see hits you with a melee attack and is adjacent to you at the time, you know the foe's location. For this reason, smart foes move right after they attack; even a foe that has made a full attack can move after attacking by taking 5-foot step (provided it has not already moved during its turn).

When an unseen foe hits you with a melee attack from more than 5 feet away, you know the general direction from which the attack came and that the attack came from more than 5 feet away, but you do not know the attacker's location.

That is the main reason to spring attack: you need to move after attacking, even if invisible, otherwise your target will know your location. If your target has failed its spot check, you still have total concealment (50% miss chance).

1. How does this work against creatures with darkvision considering they can see perfectly in dim light?

It works exactly the same for several reasons. Firstly the Shadowdancer isn't even ~in~ the area of dim light, they're standing in plain sight so normally anyone could see them. Secondly the shadows aren't what are hiding the shadowdancer, the shadows are the required medium for them to use their (Su) - Supernatural ability; note the text:

As long as she is within 10 feet of an area of dim light, a shadowdancer can hide herself from view in the open without anything to actually hide behind.

The shadows aren't hiding the shadowdancer, they're just allowing the shadowdancer the chance to use their supernatural ability, the shadows are like a focus for a spell.

2. How should I handle the being 10 ft near dim light?

For any general ability and/or targeting of supernatural things the character needs to be able to see (ie have line of sight) to the target/object/etc in question. So that is how I would suggest running it. If the character is standing by a 20' wall and the shadow is on the other side of the wall, it wouldn't work. If however there is a window in the wall then it would.

3. Since it specifically mentions that a shadowdancer cannot use her own shadow, does this mean she can use someone else's shadow even though there is no dim light nearby?

Yes. A shadow is a shadow is a shadow; unless it's the shadowdancers. Since the ability states they can't hide in their own shadow it's a pretty safe conclusion then that every other shadow is fair game. Otherwise why on earth are they called shadowdancers?

The actual size of the shadow becomes important then (blades of grass was mentioned in another question for 3.5) and I'd require that the shadow be of at least one size category smaller than the character.