Having them all roll seems silly to me. If they are on guard together I would treat them as assisting each other if they're actually doing what they're supposed to do. One check with advantage, since advantage can't stack they essentially get two checks and done. However if all the guards are chatting it up and gambling and generally distracting one check with disadvantage. Makes perfect sense to me and seems completely in the realm of the rules as presented so far.
Two rolls yeild up more information to play with IMHO, did you win by overpowering the orc's strong defenses (both roll well), or did you slip through because he moved the wrong way (both roll poor), et c.
A character tries to wrestle free from a orc's grapple. Rather than opposed strength roll, the player makes a strength roll against 10 + the strength of the orc. He fails. the orc then tries to flip the character on the ground to bash him for some damage. Orc roll strength against the character's strength +10.
Having them all roll seems silly to me.
I've come to dislike passive skills. I think they can work for monsters and NPCs as in the situation the OP describes, acting essentially as a DC for the rogue's Stealth check to beat, but I hate them on PCs because I constantly get my players saying "18 Passive Perception, are there secret doors?" or "21 Passive Insight, is he lying?" I think players (mine at least) tend to see them as the "I deserve things for free" rule. In addition, especially if you start allowing Passive Perception to spot traps/other secrets, you get a weird game where you as the DM have to either set the Perception DCs so low that everything automatically gets found by your highest-Perception PC, or so high that only that PC stands a chance of finding things. I'd rather be able to set DCs such that every player has some chance of succeeding and some chance of failing, and the best PCs just have a better chance. And in fact, that's what the randomness of Contests allows for-- the player with the highest Perception, Stealth, etc. won't necessarily be the one who succeeds every time, and the other PCs won't always fail.
Note that passive skills are a part of 5e in an oblique way. If the DC +5 is equal to or greater than the ability score you do it automatically. This could potentially allow the rogue to auto sneak past the kobolds without a roll.