Going through
@Xetheral 's questions
1.
Can a character with the Invisible condition be automatically seen by an observer with normal senses who gains line of sight to the Invisible character?
Maybe.
If the Invisible condition was gained via the Invisibility spell, then no. (Not specifying this in the spell is an oversight, in my opinion.)
If the Invisible condition was gained via Hiding, and "line of sight" means looking straight at the character (ie: no additional caveats about hanging from the ceiling vs facing on a map, or something like that), then yes. Note that "seeing" isn't the same as "finding".
(Thus ending the Invisible condition if the observer is an enemy and the condition was provided by the Hide action, but not ending the condition if the observer is not an enemy or the condition was provided by the Invisibility spell.)
With the caveat that "seeing" is not necessarily the same as "finding", this is also a "maybe" on whether it ends the Invisible condition if it was gained via Hide. In combat this will almost always count, though.
It will never end the condition if it was gained via the Invisibility spell.
If yes, what benefits do you see a hiding character as gaining from the Invisible condition that they didn't already have by being unseen by enemies (as required to be able to take the Hide action in the first place)? If no, what is required for an enemy to count as "finding" the hidden character? If your answer depends on the source of the Invisible condition, is there particular rules text that leads you to conclude that the visual appearance of the Invisible condition varies based on the source of the effect?
If the character is seen but remains Invisible via Hide, it grants the benefit of being able to move inconspicuously around, or to not be a target for the observer to confront.
To "find" a hidden character (assuming line of sight), that would either be due to the observer being an enemy who can immediately recognize the character, either as a known person, a suspicious person, or as an obvious enemy (depending on the context of where the encounter occurs), or due to a Perception check overcoming the Stealth check (such as seeing through a disguise).
In the case of the "hanging from the ceiling" scenario, a Perception check would be for the purpose of gaining proper line of sight in the first place.
2.
Do you allow characters to have the Invisible condition with respect to some creatures but not with respect to other creatures?
No. If you have the Invisible condition, it's universal.
That said, you can be seen by non-enemies and it won't affect the condition. EG: The rogue can hide behind the tree with the cleric, Hide, and gain the Invisible condition while not affecting the cleric's ability to see him, nor losing the condition just because he was seen/recognized/"found", since the cleric is not his enemy.
If no, how do you adjudicate a situation where an enemy finds a character who gained the Invisible condition by taking the Hide action (which would normally end the Invisible condition), but who cannot (or chooses not to) communicate that they found the hiding character to anyone else?
Let me reframe this into an example scenario.
OK, suppose we have the rogue, barbarian, and wizard in a side room as a larger fight rages. In this room with you is an enemy cultist. The wizard casts Silence in the room, and the barbarian grapples and restrains the cultist. The cultist can see all of you, and obviously recognizes you as enemies.
You, as the rogue, want to Hide and sneak back out into the main room, perhaps to lure another cultist into this little cubbyhole trap. Can you Hide?
Going strictly by the rules, no. You are within an enemy's line of sight, even if the ones you intend to Hide from are in the next room. However, that seems a bit absurd.
I would consider this a case where the term "enemy" needs a clarification, and that an "enemy" needs to be capable of and willing to act on its knowledge. If the enemy cannot yell or attack or move or otherwise act against the character he has "found" then he's essentially a non-entity within the scope of the scenario, not an "enemy".
While that's for "can not act", things get a little murkier for "does not act", where it varies between "will not act at all" and "will not act immediately". If you're seen by an observer who will not act at all, then that essentially is not an enemy, because no action will come from being found. However if you're seen by an observer who will watch you, but also act in the event of you starting a fight, then at the moment the observer acts, you lose the condition. Basically, there is a point where the observer transformed into an "enemy", and that's when the "found" clause kicks in.
3.
Does a character who takes the Hide action know if they have successfully obtained the Invisible condition? If yes, what (if anything) prevents a character who fails to obtain the Invisible condition from continuing to take the Hide action until they succeed, and what (if anything) prevents a character from learning that they are in line of sight to an enemy if they fail to obtain the Invisible condition despite succeeding the DC 15 check? If no, what (if anything) prevents a character from learning (whether intentionally or via happenstance) via non-enemies whether they have obtained the Invisible condition?
The character will always gain the condition if he beats the DC check while fulfilling the preconditions. If you don't meet the preconditions (based on what you know), you can't roll. If you do roll, then it's just a matter of beating the DC.
In the event of another hidden observer watching as you attempted to Hide, that gets back into the question of whether the hidden observer counts as an enemy. I would again say that when the observer acts on its knowledge, that transforms the observer into an enemy, and that's when the condition is lost.
Edit: Same thing if your cleric friend gets mind-controlled after you Hide, and yells out your position. He transformed from an ally to an enemy, and therefore was able to cancel your Invisible condition. "Enemy" is not an immutable state.
Likewise, in the other direction, an enemy that your party has used Charm Person on wouldn't count as an "enemy" for the purposes of finding you when you're hidden.
4.
What (if anything) do you do to make hiding a viable part of gameplay for low-level characters and/or parties who can't consistently make the DC 15 check required to succeed at the Hide action?
First, I'd allow a Take 10 in non-combat situations. With proficiency in Stealth, anyone with 14-16 Dex would pass at 1st or 5th level. They'd be easy to spot as soon as anyone started searching, but it at least gets things started.
If rolls are necessary, then Help and Guidance are obvious ways to improve your odds.
Beyond that, I'd probably go with per-action Stealth checks that don't result in gaining the Invisible condition. At low levels I'd likely go with only a few patrolling guards, so you just have to wait for an opportunity to move. Those moves would have much lower difficulty than a full-on Hide to become Invisible.
5.
How do you handle situations where an unseen character wants to become or remain unheard by an enemy? If you view becoming/remaining unheard as part of the Hide action, what rules text leads you to this result?
I would consider remaining unheard as part of the Hide check. Hearing is one way of using Perception, and a successful Perception check can find the hidden character, so to me, the amount of incidental noise being made is in line with the original Stealth check.
