Old Gumphrey said:
Let's see if I got this:
My gf's rogue is already hidden from a stealth roll. Combat begins. She "stealthily" attacks, beats everyone's perception roll. Sneak attack. She is still hidden from everyone's view?
No. She is considered hidden because she is already hidden. Yes, she gains combat advantage, but the attack is not "stealthy" and once she attacks she is no longer hidden.
PHB, p188: "Success: You avoid notice, unheard and hidden from view. If you later attack or shout, you're no longer hidden."
She then makes a stealth check to "stealthily" attack and fails. No sneak attack. She is now visible to all who pass the perception roll. She then drops prone into concealment (we're in the woods) and "stealthily" moves away from her position. With a roll of 30, she is now hidden from everyone's view?
As above, the attack itself cannot be stealthy. However, her crawl action can be "stealthy," and note that she gets a -5 to her roll if she moves more than 2 squares.
Note that some
monsters have the "Sniper Fire" ability, which allows them to remain hidden if they miss a ranged attack. However, this is not a character ability.
The goblin archer readies an action to shoot her the next time he sees her.
Now she "stealthily" moves behind a tree (with another 30) and then "stealthily" attacks said archer with another 30 stealth roll. He (miserably) fails his perception roll; can he attack her? Regardless of the goblin's action, her attack grants combat advantage?
Her attack has combat advantage against the goblin, but she also has a -2 to her attack due to firing while prone.
The goblin can then attack her with his readied action, because she broke her stealth when she attacked. Then, during the goblin's turn, he can move adjacent to her, and use a melee weapon to attack her with combat advantage, since she is prone.
Next, she leaves the safety of her tree, crosses a clearing, and gets behind a second tree. She cannot roll Stealth, because she entered clear line of sight?
Yes. Also remember that standing up is a move action, and if the goblin is adjacent (as he would be in my example above), moving away would provoke an Opportunity Attack.
Regardless, can she then "stealthily" drop into a crouch and become hidden from anything but clear line of sight?
Yes. However, her opponents would presumably know which square she's in, even if she is hidden.
Hope this helps.
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PS: Regarding using allies to gain cover and roll Stealth, the answer is "no."
PHB p 290: "When you make a ranged attack against an enemy and other enemies are in the way, your target has cover."
This means that the cover ONLY applies during the ATTACKER's turn,
when they attack you, and therefore you cannot duck behind your friend and make a stealth roll, since on your own turn your ally does not count as cover.
