RAW the Rogue does receive the benefits of Expertise to his passive.
A passive check is a special kind of ability check that
doesn’t involve any die rolls. Such a check can represent
the average result for a task done repeatedly, such as
searching for secret doors over and over again, or can
be used when the DM wants to secretly determine
whether the characters succeed at something without
rolling dice, such as noticing a hidden monster.
Here’s how to determine a character’s total for a
passive check: 10 + all modifiers that normally apply to the check. - Player's Handbook P.174
The player has invested in making this character good at this, this is not a bad thing.
Rogues in general are supposed to be good at this type of thing, hence the proficiency bonus class feature and the player specifically spent a feat on making his passive perception better. The player could've spent this on another feat to specialize in another area (or gain a combat ability). This is a good thing, while it may make it difficult for the Rogue to be surprised by a trap or an ambush you should in no way try to outmaneuver this.
Don't worry, there are downsides to the Rogue for this.
To reliably spot traps and ambushes the rogue will need to be at the head of the marching order so that he has clear sight lines. Rogue's are not particularly hardy and the party as a whole is inviting more risk for this reward. Likewise while he is extremely excellent at spotting physical dangers, the Rogue will not be able to detect magic wards and other dangerous enchantments and may equally blunder into them.
Ways to handle his detection of a trap or an ambush
You are correct in assuming that only that PC has seen the trap/enemy. Unless he has some mode of telepathic communication he will need to speak out and announce the threat to everyone. The best way to handle this sort of thing is to pass notes or send text messages to the player(s) able to see/detect the issue and leave it to them to react and tell someone else. Intelligent enemies will see/hear the rogue warning the party and the combat should start immediately (no surprise round though).
The PC does not become omniscient of the trap upon detecting it.
The PHB itself is very, very vague on what information is received when a PC detects a trap. However, the DMG does have a nice little section about traps, their detection, and disarming them:
If the adventurers detect a trap before
triggering it, they might be able to disarm it, either
permanently or long enough to move past it. You might
call for an Intelligence (Investigation) check for a
character to deduce what needs to be done, followed
by a Dexterity check using thieves' tools to perform the
necessary sabotage...
...In most cases, a trap's description is clear enough
that you can adjudicate whether a character's actions
locate or foil the trap. As with many situations, you
shouldn't allow die rolling to override clever play and
good planning...
...Foiling traps can be a little more complicated.
Consider a trapped treasure chest. If the chest is opened
without first pulling on the two handles set in its sides, a
mechanism inside fires a hail of poison needles toward
anyone in front of it. After inspecting the chest and
making a few checks, the characters are still unsure
if it's trapped. Rather than simply open the chest, they
prop a shield in front of it and push the chest open at
a distance with an iron rod. In this case, the trap still
triggers, but the hail of needles fires harmlessly into
the shield. - Dungeon Master's Guide p. 121
Essentially though whether trap disarming is simply a dex check, a series of checks, and/or involves serious RP is up to you as a DM. I would encourage the open-ended approach the book suggests as it adds complexity and makes trap checking and disarming a more engaging process. Sitdown with the party OOC and discuss what the table as a whole thinks should happen for trap checks and move forward based on that consensus.
There is no such time interval
If a hero successfully hides, DM is not obliged to repeat the check just because some time passes.
If there was such an interval, noticing the hidden hero was the matter of time. However, winning a Stealth/Perception contest assumes the hero can succeed (hence, stay unnoticed).
From the Core Rulebook:
Skill Checks
When your character uses a skill, he isn't guaranteed success. In order to determine success, whenever you attempt to use a skill, you must make a skill check.
So, winning the contest doesn't mean you hide "for the next N seconds",
it means the whole situation was successful. However, the situation itself might change.
For example, when your hero leaves cover, you need to make another roll if you want to remain hidden:
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.
That doesn't imply you need to make such roll when you don't leave cover.
DM should ask for another roll when circumstances change
If the hero tries to do something else except hiding, or the "bad guy" get a clue, or circumstances change (e.g. morning comes), the DM can ask for another check, if they thinks it is necessary - but the more checks they asks, the more risky situation becomes for the hero. How risky it should be, is completely up to the DM.
DC itself can depend on how long the hero intents to hide for. Hiding all day is harder than hiding for a few seconds. In both situations it will be a one single roll though.
Best Answer
TL;DR
The rogue makes a single Dexterity (Stealth) check against a DC15 passive Wisdom (Perception). Beat that and the rogue gets past.
What the rules say about hiding
Stealing large chunks from my answer to What advantages does hiding have?
Best to go back to the source on this; bold italics are my emphasis.
PHB p.177
PHB p.177
PHB Errata
PHB p.183
Note: Blindsight is dim light and results in a lightly obscured area. Torches, lanterns etc are bright light.
Being seen is not enough for you to stop being hidden - you must be seen clearly. Lighting, cover and camouflage would all influence this.
Your specific questions
... can this Rogue sneak past with a disadvantaged stealth attempt?
Yes and furthermore I can see no reason why there would be disadvantage to this. Anyone can hide where nobody can see them; the purpose of the Dexterity (Stealth) skill is to be able to hide where people can see you due to your skill, training and luck.
Do the orcs get a chance at Passive Perception to see the Rogue sneaking (using stealth skill)?
Yes - if the rogue's Dexterity (Stealth) check beats the orc's passive Wisdom (Perception) then he sneaks past them.
Do we roll/check for the Sneak and the Orcs get to check Passive Perception?
Exactly.
The Rogue's speed is 25 feet (Dwarf/light armor), or 6 squares on the map and that gets him right near the orcs, but not past them, so the orc should get a Passive Perception check before the Rogue gets to move another 6 squares past them. Is this correct?
No, because:
25 feet is 5 squares on the map (just saying).
You do not use round by round movement unless you are in a combat situation so if the rogue sneaks past then he sneaks past, if he doesn't you may be rolling initiative and switching to round by round movement then.
There is no passive [Wisdom] (Perception) check - it is a static number; for an orc it is 10.
Effectively the rogue makes a Dexterity (Stealth) check against a DC of 15 (10 for orc's passive Wisdom (Perception) + 5 for advantage because there is more than one orc doing the looking and they therefore help one another), if the area is brightly lit and there is no cover.
If there are sufficient objects/people to hide behind or it is dimly lit (which includes darkvision) then the DC is 10 as the disadvantage for lightly obscured cancels the advantage from helping.
If the rogue is doing it in the dark (and he can see somehow) then it is automatic; barring making a noise.
Why not use active Wisdom (Perception)
Because the orcs are "standing guard" which to me does not engage "actively searches for signs of your presence." Standing guard is long, boring duty and you are not going to pay attention all the time especially when you have a couple of mates around to chat with.
If they were in a state of heightened alertness (like they knew you were out there right now!) then they could use active Wisdom (Perception).