(Yeah, that makes the most sense to me.)

“If you can take me close to them, maybe to a rooftop, I may be able to compel them to stop.” 

Titus scans the skyline for a spot for Rethan to teleport them that might be within 30feet of the devils trajectory.

Perception 10

Rethan stares out into the city, anticipating where the spined devils are heading. He claps a hand onto Titus' shoulder and, with a thought...

...transports the both of them to the roof of a building 77 feet in front of the flying devils.

(Titus may go next, then the spined devils and Laniel will go.)

Titus will hold his turn, he keeps a close eye on the devils and will wait until they fly closer before he acts. 

(Hopefully they keep flying this way!)

DM shield:

The spined devils do not realize that Rethan and Titus are pursuers until it is too late, flying 10 feet over the rooftops, lifting slightly as they fly overhead. But they are flying fast, and before Titus has a chance to act, they have zipped past, to nearly 130 feet beyond where Rethan and Titus appear.

(Titus and Rethan are 129 feet behind the spined devils now.)

Laniel, still back at the windmill, races out into the streets, pushing himself as fast and hard as he can towards the giant floating arrow hanging over the rooftops. "Well done, Morph," he mutters as he runs.

DM Shield:

After racing nearly 250 feet through the streets, Laniel encounters a patch of water and oil that a merchant has carelessly tossed into the street.

DEX check: 14

But the slick ground proves to be no obstacle as Laniel slides effortlessly over it and continues the chase.

Laniel presses himself hard, trying to catch up.

CON check: 12

And finds plenty of reserves to run full tilt after the devils.

(Laniel is 260 feet behind the spined devils. Everyone else may continue the chase!)

Mal will go all out, calling on inner reserves he wasn't sure he had, but was willing to put to the test

Mal will dash 8 times in an attempt to cover lost ground, following Morph's arrow.

Con check 1: 6 LUCK!!!: 4

Con check 2: 9 LUCK!!!: 5

Con check 3: 17 

DM shield:

After sprinting as hard as possible, Mal gains a fair amount of ground on the devils. Unfortunately, as he runs down a long stretch of alley, he comes to a series of clotheslines strung across the alley, draped with wet clothes. Mal pushes to burst through them but finds he has run out of energy and smacks off of them as if a solid wall. He falls flat on his butt, panting.

(Mal has fallen prone.)

Morph keeps up his pursuit, pulling the arrow along with him.

Morph flies over Rethan and Titus, not seeing anyway he can help them, he keeps going.

Concentration: Silent Image

Action: Move the illusion to place it 60ft above Morph again.

Move: 50ft towards the devil

Bonus: Saved for special dashes

Dashes: 3 + 0 (CON) = 150ft towards the devil

Total movement: 200ft towards the devils

DM Shield:

Morph continues to match the speed of the spined devils, not letting them gain even an inch. He is concentrating so hard on keeping up and maintaining his spell that he nearly doesn't notice the chimney in front of him at all. He rolls to the side at the last minute, feeling his sleeve graze the bricks as he whizzes past but manages to avoid injury.

(Titus and Rethan are next. The rooftops are close enough together here that they can pursue on foot without having to make any athletics or acrobatics checks for jumping. Assume all jumping distances are manageable unless I roll otherwise on the encounter table.)

Quote from MindDetective >>

The spined devils do not realize that Rethan and Titus are pursuers until it is too late, flying 10 feet over the rooftops, lifting slightly as they fly overhead. But they are flying fast, and before Titus has a chance to act, they have zipped past, to nearly 130 feet beyond where Rethan and Titus appear.

(Does this lift them above my 10 foot reach with my glaive?)

(No, it does not! You should be able to hit them.)

Quote from MindDetective >>

(No, it does not! You should be able to hit them.)

 (In that case, opportunity attack from polearm master with the glaive when they get in range of me :).  Against the one carrying the key)

Attack: 11 Damage: 9

Rethan takes a swipe at the lead spined devil but his blade barely misses the fiend as it passes by.

Rethan spins and dashes after the spined devils as they fly overhead.  As he does so, Midnight appears ahead of him and darts out after the devils as well, quickly gaining a lead over Rethan himself.  If he manages to close the distance enough, he launches a javelin at the key-carrier.

(I can dash 5 times without a con check, which gives me up to 300 feet of movement.  Will add 2 more since they're faster than me to try to close some distance, for 420 feet.  Con saves: 11, 5.  I'm assuming with the chase rules it's assumed we're all moving at the same time (hence the distinction on attacks of opportunity while chasing), not moving one at a time like in normal combat, so can only attack if I manage to close the distance compared to their own movement)

(129 feet behind, 120 foot long range on the javelin, so if I close at least 10 feet closer, should be in rnage for the javelin)

If needed: Attack: 25 Damage: 7, disadvantage for long range 8

(Pretty much, yes, on AoO. Five dashes should only be 150 feet, though, right? 30 feet per move and dash? Two more gets you to 210 feet. But that vaults you way past them, since they are only 129 feet ahead of you. You don't need the extra burst of speed here. In this case, you teleported in, began jogging in the same direction, then took a swing as they flew overhead. So you can take an actual attack at them instead of the javelin, if you prefer, although the javelin will hit, since disadvantage isn't necessary.)

Quote from MindDetective >>

(Pretty much, yes, on AoO. Five dashes should only be 150 feet, though, right? 30 feet per move and dash? Two more gets you to 210 feet. But that vaults you way past them, since they are only 129 feet ahead of you. You don't need the extra burst of speed here. In this case, you teleported in, began jogging in the same direction, then took a swing as they flew overhead. So you can take an actual attack at them instead of the javelin, if you prefer, although the javelin will hit, since disadvantage isn't necessary.)

 

 (Oh, I did the math wrong, thinking dash = double move speed (since that's basically what it does in normal combat) and took that 30*2 times the number of dashes.  Excet dash doesn't actually double your move speed, it just lets you move your movespeed again, so yah, 5 dashes + 30 foot base speed would end up being 180 feet (150 for dashes, + 30 base).  For the rest, that's where I wasn't sure if we were still taking turns like in normal combat, or if w eeffectively moved all at the same time (so if they're 129 feet ahead and they move 160 feet while I move 180 feet, I only close the gap by 20 feet, which gets me to 109.  Versus taking turns where I move 120 feet closer, then attack them, then they move 160 feet away (or whatever distance).  The extra burst of speed was based on the former, and needing to go faster to catch up (based on wording for opportunity attacks where you're assumingt o be moving the same direction together).  if it's the later and I can get in range to attack, I'll just use the glaive instead of the javelin, which would be 1 damage with the same attack modifier.

(There's no great way to do this except in rounds. *shrug* And I think your damage there was a 4...+4, so 8? AND, you could give up a dash action to use your bonus attack ability to strike again, if you like. This means only moving 150 feet, but that is still more than enough.)

Quote from MindDetective >>

(There's no great way to do this except in rounds. *shrug* And I think your damage there was a 4...+4, so 8? AND, you could give up a dash action to use your bonus attack ability to strike again, if you like. This means only moving 150 feet, but that is still more than enough.)

 (Er, yah, 8 damage, forgot the +4 bonus.  And yah, if I don't need the fifth dash, bonus attack it is: Attack: 17 Damage: 6)

Rethan dashes across the rooftops, making graceful leaps as gaps appear, keeping stride with the devils. When the timing is right, he lashes out with his glaive, striking the lead devil. Then he twirls the weapon and jabs up at the fiend again, finding only air.

DM shield:

As Rethan finishes his swing, he encounters a pile of boards with their nails sticking straight up in a dangerous fashion. He leaps over them at the last minute, rolling and standing again to continue the pursuit.

Titus may go, then Laniel and the devils will go.

If I’m reading the chase rules right, Titus can dash five times with no CON check, and I believe this can get him within 30ft, yes? If so...

Titus rushes after his quarry and after barreling through the streets he bellows out: “Stand and face me devils! In the name of Waterdeep and those who serve her, I we deliver justice unto you!”

Champion Challenge - As a bonus action, each creature of your choice within 30 ft. must make a WIS saving throw (DC 13) or it can't willingly move more than 30 ft. away from you.

If I read that wrong and Titus cannot use his champions challenge this round he will just close as much distance as he can.