pming
Legend
Hiya.
1a = Yes.
1b & 1c = Irrelevant. The Rogue gets to actually make a Stealth roll because he's trying to hide in the bushes. Without bushes, he'd be standing in plain sight and the orc would see him, obviously. In this case, I'd rule that the "light obscurement" doesn't impose any disadvantage to the orc...it simply gives the rogue somewhere to hide in the first place. Now, add in a light fog/mist...now the rogue gets the light obscurement benefit (e.g., the orc has Disadvantage/-5 to his detection chances). Is this "as per the rules"? *shrug* Doesn't really matter does it? As long as I (the DM) am consistent in how I rule this, my game will run just fine.
2a = Yes. He's now actively looking, not passively.
2b = Nope. Hiding in the bushes actually gives the rogue a roll in the first place; ignore the whole "lightly obscured" rules as written for this because otherwise it would be silly and make no sense. Luckily, I'm human and can think "outside the box".
Not really...I'm just interpreting them differently. Nothing wrong with that, is there?
As I'm interpreting this situations "light obscurement" differently, these rules aren't going to apply as the are written, so...uh...
I guess whenever he has something to hide behind? I mean, if someone is trying to interpret semi-vague rules as strictly as possible, that person is going to be exceedingly frustrated. If a rule is "vague" then it needs to be treated that way; you can't treat it as "iron-clad" or a "yes/no". Pretty much all of 5e is written with this "DM/Player interpretation" rule style. The Stealth rules are, as Mearls said, written that way on purpose; that purpose being for individual DM's to decide the specifics simply because there are FAR to many variables to write rules that would work in every situation in a RPG. A DM can adapt to the situation...rules written on the printed page can not. It really is that simple.
Stop scratching your head about it. Think about how you want them to work in your game and implement them that way. If they don't work for you, change them until they do. This may be as simple as copy/pasting rules from some other d20 supplement/game that you DO like...or maybe you will need to write some stuff out yourself for your campaigns house rules.
I keep seeing threads about Stealth/Hiding, and, as I've said a million times before...stop thinking in 3.x/PF/4e terms. In those games, you had rules with pages of modifiers, with yes/no specifics, and then you had a bajillion exceptions that broke those "iron clad" rules/modifications in terms of spells, feats, class abilities, powers, etc. In short, there was a LOT of wasted time, effort and space trying to codify "stealth" rules and then find ways to break/ignore them. Why bother in the first place? IMHO, 5e's "vague" rules are infinitely better than having 5 pages of modifiers to a die roll and another 3 pages dedicated to exceptions. With 5e, the premiere rule is this: "If you're confused, ask your DM. If you're the DM and can't find something or something doesn't make sense, make it up".
^_^
Paul L. Ming
If the rules were not vague and not crystal clear to everyone, then why did you feel compelled to write up a long post on it?
An Orc is strolling down a trail and not actively searching.
A Rogue is hiding in light obscurement.
1a) Does the Orc get a passive perception vs. the stealth check of the Rogue?
1b) Or does the Orc get a passive perception (at -5 for light obscurement) vs. the stealth check of the Rogue?
1c) If so and the Rogue cannot hide behind cover, during what type of hiding does the Orc get a normal passive perception vs. the stealth check of the Rogue?
1a = Yes.
1b & 1c = Irrelevant. The Rogue gets to actually make a Stealth roll because he's trying to hide in the bushes. Without bushes, he'd be standing in plain sight and the orc would see him, obviously. In this case, I'd rule that the "light obscurement" doesn't impose any disadvantage to the orc...it simply gives the rogue somewhere to hide in the first place. Now, add in a light fog/mist...now the rogue gets the light obscurement benefit (e.g., the orc has Disadvantage/-5 to his detection chances). Is this "as per the rules"? *shrug* Doesn't really matter does it? As long as I (the DM) am consistent in how I rule this, my game will run just fine.

Now a different Orc shouts out that someone is in the bushes. The Orc is now warned and alert, but the Rogue is still hidden from this Orc (but not the other Orc).
2a) Does the first Orc now get a perception check vs. the stealth check of the Rogue?
2b) Or does the first Orc get a disadvantaged perception check due to lightly obscured vs. the stealth check of the Rogue?
2a = Yes. He's now actively looking, not passively.
2b = Nope. Hiding in the bushes actually gives the rogue a roll in the first place; ignore the whole "lightly obscured" rules as written for this because otherwise it would be silly and make no sense. Luckily, I'm human and can think "outside the box".

If you answer:
1a) Yes.
1b) No.
1c) Then you need no answer for 1c.
2a) Yes.
2b) No.
Then you are ignoring the lightly obscured rules for disadvantage.
Not really...I'm just interpreting them differently. Nothing wrong with that, is there?
If you answer:
1a) Yes.
1b) No.
1c) Then you need no answer for 1c.
2a) No.
2b) Yes.
Then it's harder to find someone when you know that they are there than if you do not know they are there.
As I'm interpreting this situations "light obscurement" differently, these rules aren't going to apply as the are written, so...uh...
If you answer:
1a) No.
1b) Yes.
1c) Then you need an answer for 1c.
2a) No.
2b) Yes.
Then you need an answer for 1c. If the Rogue cannot hide behind cover (like a chair) as you claimed, when can he hide and have normal passive perception rolls against his stealth?
I guess whenever he has something to hide behind? I mean, if someone is trying to interpret semi-vague rules as strictly as possible, that person is going to be exceedingly frustrated. If a rule is "vague" then it needs to be treated that way; you can't treat it as "iron-clad" or a "yes/no". Pretty much all of 5e is written with this "DM/Player interpretation" rule style. The Stealth rules are, as Mearls said, written that way on purpose; that purpose being for individual DM's to decide the specifics simply because there are FAR to many variables to write rules that would work in every situation in a RPG. A DM can adapt to the situation...rules written on the printed page can not. It really is that simple.
Or is your answer that he cannot hide behind cover or behind lightly obscured areas, and can only hide while invisible or behind total cover or behind heavily obscure areas? If so, then hiding really sucks.
Sorry, but this stuff is still vague. If you could clear up these particular questions, I would appreciate it because I have been scratching my head over them.
Stop scratching your head about it. Think about how you want them to work in your game and implement them that way. If they don't work for you, change them until they do. This may be as simple as copy/pasting rules from some other d20 supplement/game that you DO like...or maybe you will need to write some stuff out yourself for your campaigns house rules.
I keep seeing threads about Stealth/Hiding, and, as I've said a million times before...stop thinking in 3.x/PF/4e terms. In those games, you had rules with pages of modifiers, with yes/no specifics, and then you had a bajillion exceptions that broke those "iron clad" rules/modifications in terms of spells, feats, class abilities, powers, etc. In short, there was a LOT of wasted time, effort and space trying to codify "stealth" rules and then find ways to break/ignore them. Why bother in the first place? IMHO, 5e's "vague" rules are infinitely better than having 5 pages of modifiers to a die roll and another 3 pages dedicated to exceptions. With 5e, the premiere rule is this: "If you're confused, ask your DM. If you're the DM and can't find something or something doesn't make sense, make it up".
^_^
Paul L. Ming