Ok, stealth in combat is an ambiguous issue as written in the PHB and you're likely to get a bunch of different answers from different people. Go with what sounds most within the intent of the rules to you and don't pay too much attention to everybody else's interpretations but your own (especially, no offense to anyone, those claiming that their own interpretations are obvious and explicitly stated in the rules etc) until an official errata or clarification is published somewhere.
That said, this is how I, personally, have interpreted the rules in the PHB after much discussion and debate with other DMs. I believe they follow the original intent of the rules and are fair to the rogue.
No, the section of "ally-cover" states that
"When you make a ranged attack against an enemy and other enemies are in the way, your target has cover."
Therefore your alliy counts as cover only for the purposes of determining the enemy's ranged attack against you, nothing else.
No, if you have cover against one or several enemies, and you take an action which you wnt to take stealthily, you make an opposed Stealth roll versus their Active Perception (if the creature is currently actively fighting you) or their Passive Perception (if the creature is fighting someone else or otherwise not focused particularly on you), and are then hidden to that creature.
Yes, once you have cover (and only after you've achieved cover) you can take a stealhy action to attempt to hide.
Note that there is no actual 'hide' action, as in 'this is me hiding now'. You roll as Stealth roll as part of another action which you want to do stealthily. If you want to hide in a tree, first you need to move behind the tree and gain cover from an enemy, then you would try to climb this tree with a Stealth roll included in the action, and if you pass it, you are then hidden up in the tree for and and all creatures you beat with the roll.
If you want to hide among some bushes, you would need to first enter the bushes to get cover, then with another separate move action move inside the bushes to another location, but this time stealthily, so that now you are hidden in a spot noone saw you go into, therefore they are unaware of your exact location and you can get CA against whoever failed their opposed Perception.
Once you attack an enemy, that enemy becomes aware of you and you can't sneak-attack that particular enemy from that particular location any more, you have to either stealthily attack someone else or stealthily move somewhere else (but it can be as simple as just moving to the next square over that has cover).
Whether you can take a Stealth roll with the very action that moves you under cover is debatable, I have interpreted that no, you can't do that, since you don't have cover at the beginning or middle of that move. You must have cover to attempt perform an action stealthily.
(Yes, this means that you can't become 'hidden' when an object provides a single square of cover, since you can't move around within the covered area. So be it.)
Actually, with a well-built area rogues are plenty happy with hiding options. The Kobold lair outside is filled with trees that grant concealment, for instance. The other two starting encounters also have plenty of areas which grant it. Our rogue had lots of fun running around in them 
The Inner Kobold Lair has a bunch of free-standing walls, our rogue just hid behind the cover of the center wall (on the north side) and walked over to the other corner and started taking sneal-attack potshots at the kobolds on the southside (while the paladin kept the kobolds up on the north side busy).
That said, yes the rogue will be much more effective once he gets a few concealment-granting abilities under his belt.
Nope, ally-cover does not count as cover for purposes of hiding. But it's a good question that has popped up several times.
As I said in the beginning, this is interpretation, but it;s the interpretation that I like the most, makes the most logical sense to me, and seems most in line with what is intended of Stealth. Many others' interpretations may vary, and they are probably fine too, until we get some official message on this.
