Much of your question is handled in How does Stealth work with Sneak Attack? The additional circumstances. similarly, need GM discretion and interpretation of RAI - RAW doesn't work.
Vision and Light
Let's look at these rules.
In an area of bright light, all characters can see clearly. Some creatures, such as those with light sensitivity and light blindness, take penalties while in areas of bright light. A creature can't use Stealth in an area of bright light unless it is invisible or has cover. Areas of bright light include outside in direct sunshine and inside the area of a daylight spell.
Normal light functions just like bright light, but characters with light sensitivity and light blindness do not take penalties. Areas of normal light include underneath a forest canopy during the day, within 20 feet of a torch, and inside the area of a light spell.
In an area of dim light, a character can see somewhat. Creatures within this area have concealment (20% miss chance in combat) from those without darkvision or the ability to see in darkness. A creature within an area of dim light can make a Stealth check to conceal itself. Areas of dim light include outside at night with a moon in the sky, bright starlight, and the area between 20 and 40 feet from a torch.
Notice how "normal light functions just like bright light except." The RAW reading of this is:
"In an area of normal light, all characters can see clearly.
Some creatures, such as those with light sensitivity and light blindness, take penalties while in areas of bright light. A creature can't use Stealth in an area of normal light unless it is invisible or has cover."
Thus you can't use stealth in bright or normal light and you can't sneak attack in dim light or darkness due to the concealment. Voila, RAW means you can never use Sneak Attack unless you're invisible. Do you think that's the rules' intent? If so, stop here.
So you're still with me. There's been recent clarifications on the whole "jumping out to sneak attack" thing but the PF devs have been restricted because their changes can't change the pagination in the book. But from their discussion (see the first linked question), it appears to be the intent that people, in normal conditions, can Stealth and then pop out and stab someone.
In the full/normal light situation, yes, you can't hide/initiate a Stealth without cover or concealment. But in terms of then moving out to stab them, you're deliberately getting a "bye" on ALL the stealth exceptions including "full light." You can't Stealth without concealment either, but you don't have any, you left it.
The new errata, after most of the "full light" arguments on the Paizo boards, says
Breaking Stealth: When you start your turn using Stealth, you can leave cover or concealment and remain unobserved as long as you succeed at a Stealth check and end your turn in cover or concealment. Your Stealth immediately ends after you make an attack roll, whether or not the attack is successful (except when sniping as noted below).
If you're Stealthed, you can pop out and stab someone and then become visible. Note it doesn't say you become observed as soon as you take "an action" - it says an attack. If you pop out, move up to someone, stab them you are still benefiting from your initial Stealth till the stab. Or if you pop out and stop and do something else in the open, your Stealth is gone once your turn is over. If you go from stealth area to stealth area, you're OK, regardless of light. More backup for this position...
Jason also says in that same thread,
The wording was intentionally put together to specify "at the end of your turn". That is the moment when you check your status to see if you can maintain Stealth. This does allow you to move from cover, use Stealth to approach a target, and make a single attack, at which point, Stealth is broken, regardless of the outcome.
So no you can't Stealth out in the open in bright light (or normal light!) but that has zero impact on your ability to Stealth in cover, leave that cover, walk up and get a first SA stab in on them. Light levels only affect the stealthing/Perception of the prior Stealth attempt (and subsequent one, if there is one attempted at the end of that round).
Other PCs' actions interrupting/affecting others has always been a weird edge case in the rules. Normally, the detection and shouting needs to happen on someone's turn. Unless they have a held action, that's not really going to happen during the potential victim's turn. Now, it's fair and it happens for one PC to see a hidden guy and yell "hidden thief over there behind the pillar!" By the normal action rules, who knows what happens, probably nothing. I'd allow a new Perception check for the allies assisted by the smartypants. If they still don't see him, though, they don't see him - maybe they're looking at the wrong pillar, maybe they misunderstood, I know that in the real world I've seen plenty of "look out for that thing!" that ended in someone getting bonked in the head by that same thing.
In the really unlikely event someone has a held action waiting for the rogue to break Stealth to yell a warning to their buddy - fair enough, that's certainly a good use of an action, I'd let them get a Perception check with quite a bonus!
Rulings over Rules
Anyway, if you're a new Pathfinder GM - do what makes sense. Don't go down the rabbit hole of worrying about what exactly the RAW is, even the game designers say "well if it seems like there's a paradox or obvious wrong thing, why would you do that? Do what makes sense." It's remarkably hard to infallibly describe even simple human activities like "seeing something;" they provide some guidance but expect you to fill in the gaps.