Alrighty then. Well, first off, welcome to the forums!!
Now as to your question here's how it breaks down.
The only time you can shoot at someone who is in Cover is if they are the closest enemy. For someone who has Stealth, they are NEVER considered the closest enemy when in Cover and more than 6 squares away.
So if the only person you can see has Stealth and is in Cover, you would normally not be able to shoot at them (assuming they are more than 6 squares away).
Your 2nd question isn't very clear to me. I am guessing you are asking if there are two people you can see where one has Stealth and the other doesn't, who can you shoot at. Let's assume that the enemy without Stealth is farther away than the enemy with it. Since the enemy with Stealth cannot be targetted while in Cover when >6 squares away, they are not considered a legal target. So that gives you the ability to shoot at the guy without Stealth (this is because the Stealthed character is not considered the closest enemy even when physically he is).
The way Accurate Shot plays into this is that special ability allows a character to make an attack against a target in Cover even if they aren't the closest. Basically, Accurate Shot allows you to shoot at people who have Stealth, or other characters hanging out in the back.
Here's a link that's MUCH more detailed.
http://forums.gleemax.com/showthread.php?t=1123262Hope that helps.
Sincerely,
Jester007